TPG

Question 1

What is Python?

Answer 1

Python is a general-purpose programming language created by Guido Van Rossum. Python is most praised for its elegant syntax and readable code, if you are just beginning your programming career Python suits you best.

With Python you can do everything from
GUI (graphical user interface ) development,
Web application,
System administration tasks,
Financial calculation,
Data Analysis,
Visualization
and list goes on…

Question 2

What doeas it mean that Python is interpreted language?

Answer 2

Język interpretowany – język programowania, w którym interpreter analizuje program linia po linii. Przeciwieństwem języków interpretowanych są języki implementowane w postaci kompilatora (kompilator najpierw kompiluje cały program, a następnie zaczyna działać). Języki interpretowane są nieco wolniejsze od języków kompilowanych, lecz prostsze do napisania (mniej rzeczy o które trzeba się martwić podczas pisaniaskodu)

Python is interpreted language, when you run python program an interpreter will parse python program line by line basis, as compared to compiled languages like C or C++, where compiler first compiles the program and then start running.

Now you may ask, so what’s the difference??

The difference is that interpreted languages are a little bit slow as compared to compiled languages. Yes, you will definitely get some performance benefits if you write your code in compiled languages like C or C++.

But writing codes in such languages is a daunting task for a beginner. Also in such languages, you need to write even most basic functions like calculate the length of the array, split the string etc. For more advanced tasks sometimes you need to create your own data structures to encapsulate data in the program. So in C/C++ before you actually start solving your business problem you need to take care of all minor details. This is where Python comes. In Python, you don’t need to define any data structure, no need to define small utility functions because Python has everything to get you started.

Moreover, Python has hundreds of libraries available at https://pypi.python.org/ which you can use in your project without reinventing the wheel.

Question 3

What does it mean that Python is Dynamically Typed ?

Answer 3

Python sam automatycznie rozpoznaje typ zmiennych na podstawie kodu.

Python doesn’t require you to define variable data type ahead of time. Python automatically infers the data type of the variable based on the type of value it contains.

For e.g:

myvar = “Hello Python”
The above line of code assigns string “Hello Python” to the variable myvar, so the type of myvar is string.

Note that unlike languages like C, C++ and Java, in Python you do not need to end a statement with a semicolon (;).

Suppose, a little bit later in the program we assign variable myvar a value of 1 i.e

myvar = 1
Now myvar variable is of type int.

Question 4

What does it mean that Python is strongly typed?

Answer 4

Nie dokonuje on automatycznej konwersji kodu.
If you have programmed in PHP or javascript. You may have noticed that they both convert data of one type to another automatically.

For e.g:

In JavaScript

1 + “2”
will be ‘12’

Here, before addition (+) is carried out, 1 will be converted to a string and concatenated to “2”, which results in ‘12’, which is a string. However, In Python, such automatic conversions are not allowed, so

1 + “2”
will produce an error.

Question 5

Which code will be longer, the one written in Python or Java?

Answer 5

Programs written in Python are usually 1/3 or 1/5 of the Java code. It means we can write less code in Python to achieve the same thing as in Java.

Question 6

Who uses Python?

Answer 6

Python is used by many large organizations like Google, NASA, Quora, HortonWorks and many others.

Okay, what I can start building in Python?

Pretty much anything you want. For e.g:

GUI applications.
Web apps.
Scrape data from websites.
Analyse Data.
System administration utilities.
Game Development.
Data Science
and many more …

Question 7

Do I have to install Python on my PC?

Answer 7

Mac also comes with python 2 and python 3 installed (if not see this link for instructions), but this is not the case with windows. Similarly, most Linux distribution for e.g Ubuntu 14.04 comes with python 2 and 3 installed, bu you have to install Python on Windows

Question 8

How to install Python on Windows/Ubuntu/Mac?

Answer 8

To install python you need to download python binary from https://www.python.org/downloads/, specifically we will be using python 3.4.3 which you can download from here. While installing remember to check “Add Python.exe to path”

If you are using Ubuntu 14.04 which already comes with python 2 and python 3, you need to enter python3 instead of just python to enter python 3 shell. Also you will need a text editor to write Python programs, you can use text editor like notepad. If you want to use full-fledged text editor then use notepad++ or sublime text. Download and install text editor of you choice.

On Mac is good to check Python version by typing in terminal: ~ % python3 –version
Then to install Python 3 with the Official Installer from: https://www.python.org/downloads/

Question 9

How to run python programs?

Answer 9

You can run python programs in two ways, first by typing commands directly in python shell:
Np.
print(“Hello World”)
> Hello World

or run program stored in a file. But most of the time you want to run programs stored in a file.
Np. do terminala wpisujemy:
python hello.py
W ten sposób wgrywamy plik z zapisanym tam uprzednio programem.

Question 10

How many arguments does print() function have?

Answer 10

as many arguments as you provide it with (as many as editor allow). When two or more arguments are passed, the print() function displays each argument separated by space.
Np.
print(“Hello World”)
> Hello World

print(“Hello”, “World”)
> Hello World

Question 11

Which command is used to change directory?

Answer 11

cd ‘od change directory’
Open terminal and change current working directory to C:\Users\YourUserName\Documents using cd command

Question 12

What to do when you want to know more about some method or functions?

Answer 12

Type help() into the terminal.
Sooner or later while using python you will come across a situation when you want to know more about some method or functions. To help you Python has help() function, here is how to use it.

Syntax:

To find information about class: help(class_name)

To find more about method belong to class: help(class_name.method_name)

Now suppose you want to know arguments required for index() method of str class, to find out you need to type the following command in the python shell: help(str.index)

Question 13

What are variables in Python and what are the rules of naming them?

Answer 13

Variables are named locations that are used to store references to the object stored in memory. The names we choose for variables and functions are commonly known as Identifiers. In Python, Identifiers must obey the following rules.

1. All identifiers must start with a letter or underscore (_), you can’t use digits. For e.g: my_var is a valid identifier but 1digit is not.
2. Identifiers can contain letters, digits and underscores (_). For e.g: error_404, _save are valid identifiers but $name$ ($ is not allowed) and #age (# is not allowed) are not.
3. They can be of any length.
4. Identifiers can’t be a keyword. Keywords are reserved words that Python uses for special purposes). The following are keywords in Python 3.

False class finally is return
None continue for lambda try
True def from nonlocal while
and del global not with
as elif if or yield
pass else import assert
break except in raise

Question 14

Values or literals?

Answer 14

Values = literals

They are basic things that programs work with. For e.g: 1, 11, 3.14, “hello” are all values.
They are also commonly known as literals.

They can be of different types for e.g 1, 11 are of type int, 3.14 is a float and “hello” is a string.

Question 15

What is a object in Python?

Answer 15

Remember that in Python everything is object even basic data types like int, float, string,

Question 16

How to assign value to a variable?

Answer 16

To assign value to a variable equal sign (=) is used. The = sign is also known as the assignment operator.

Question 17

Should I declare types of variables while using Python?

Answer 17

In Python, you don’t need to declare types of variables ahead of time. The interpreter automatically detects the type of the variable by the data it contains.

Question 18

Show some examples of variable declaration

Answer 18

x = 100 # x is integer
pi = 3.14 # pi is float
sentence = “python is great” # sentence is string

a = b = c = 100 # this statement assign 100 to c, b and a.

Question 19

What the variable stores?

Answer 19

When a value is assigned to a variable, the variable doesn’t store the value itself. Instead, the variable only stores a reference (address) of the object where it is stored in the memory.
Therefore, in the x = 100, the variable x stores a reference (or address) to the 100 ( an int object ). The variable x doesn’t store the object 100 itself.

Question 20

Comments

Answer 20

Comments are notes which describe the purpose of the program or how the program works. Comments are not programming statements that Python interpreter executes while running the program. Comments are also used to write program documentation. In Python, any line that begins with a pound sign (#) is considered a comment.
e.g:
#This program prints “hello world”
print(“hello world”)

We can also write comments at the end of a statement:
print(“hello world”) # display “hello world”

Question 21

What are end-line comments?

Answer 21

Comments that appear in this form:
print(“hello world”) # display “hello world”

Question 22

simultaneous assignment

Answer 22

The simultaneous assignment or multiple assignment allows us to assign values to multiple variables at once. The syntax of simultaneous assignment is as follows:
var1, var2, …, varn = exp1, exp2, …, expn

e.g:
a, b = 10, 20
print(a)
> 10
print(b)
>20

Simultaneous assignments is quite helpful when you want to swap the values of two variables.
e.g:
x = 1 # initial value of x is 1
y = 2 # initial value of y is 2

y, x = x, y # assign y value to x and x value to y

print(x) # final value of x is 2
print(y) # final value of y is 1
> 2
1

Question 23

Name Python data types

Answer 23

1. Numbers
2. String
3. List
4. Tuple
5. Dictionary
6. Boolean

Question 24

List values in Python, which are considered as false

Answer 24

Boolen False
0 - zero , 0.0
[] - empty list ,
() - empty tuple ,
{} - empty dictionary ,
‘ ‘ - empty string
None

Question 25

How to receive input from the console?

Answer 25

The input() function is used to receive input from the console.
The input() function accepts an optional string argument called prompt and returns a string.

Note that the input() function always returns a string even if you entered a number. To convert it to an integer you can use int() or eval() functions.

Question 26

eval()

Answer 26

Python’s eval() allows you to evaluate (parse, compile, evaluate and return) arbitrary Python expressions from a string-based or compiled-code-based input.
e.g.
»> eval(“2 ** 8”)
256

Question 27

Python modules

Answer 27

Python organizes codes using modules. Python comes with many built-in modules ready to use for
e.g
there is a ‘math’ module for mathematical related functions,
‘re’ module for regular expression,
‘os’ module for operating system related functions and so on.

Question 28

How to import module?

Answer 28

To use a module we first import it using the import statement. Its syntax is as follows:

import module_name

We can also import multiple modules using the following syntax:

import module_name_1, module_name_2

Question 29

Regular expression

Answer 29

A Regular Expression (RegEx) is a sequence of characters that defines a search pattern. For example:
^a…s$
The above defines a RegEx pattern. The pattern is: any five letter string starting with a and ending with s

Python has a module named re to work with RegEx. Here’s an example:
e.g
import re

pattern = ‘^a…s$’
test_string = ‘abyss’
result = re.match(pattern, test_string)

if result:
print(“Search successful.”)
else:
print(“Search unsuccessful.”)

Question 30

How to access objects in a module?

Answer 30

e.g
import math, os

print(math.pi)
print(math.e)

print(os.getcwd())

In this listing, the first line imports all functions, classes, variables and constants defined in the math and os module. To access objects defined in a module we first write the module name followed by a dot (.) and then the name of the object itself. (i.e class or function or constant or variable). In the above example, we are accessing two common mathematical constants pi and e from the math math. In the next line, we are calling the getcwd() function of the os module which prints the current working directory.

Question 31

What numerical types are supported in Python?

Answer 31

Python supports 3 different numerical types.

1) int - for integer values like 1, 100, 2255, -999999, 0, 12345678.

2) float - for floating-point values like 2.3, 3.14, 2.71, -11.0.

3) complex - for complex numbers like 3+2j, -2+2.3j, 10j, 4.5+3.14j.

Question 32

Complex number

Answer 32

As you may know complex number consists of two parts real and imaginary, and is denoted by j. You can define complex number like this:

> > > x = 2 + 3j # where 2 is the real part and 3 is imaginary

Question 33

How to determine variable type?

Answer 33

Python has type() inbuilt function which is use to determine the type of the variable.
e.g
x = 12
type(x)
> <class ‘int’>

Question 34

Name Python operators

Answer 34

Name Meaning Example Rresult
+ Addition. 15+20. >35
- Subtraction. 24-3. >21.0
* Multiplication 15*4 >60
/ Float Division. 4/5. >0.8
// Integer Division. 4//5. >0
** Exponentiation. 4**2 >16
% Remainder. 27%4. >3

Question 35

Integer division

Answer 35

Integer Division (//) : The // perform integer division i.e it will truncate the decimal part of the answer and return only integer.

Question 36

Float division

Answer 36

Float Division (/) : The / operator divides and return result as floating point number means it will always return fractional part. e.g
3/2
>1.5

Question 37

Exponentiation operator

Answer 37

Exponentiation Operator (**) : This operator helps to compute a^b (a raise to the power of b). Let’s take an example:
e.g
2 ** 3 # is same as 2 * 2 * 2
> 8

Question 38

Remainder operator

Answer 38

Remainder operator (%, reszta) : The % operator also known as remainder or modulus operator. This operator return remainder after division. For e.g:
> 7 % 2
1

Question 39

Operator precedence

Answer 39

Pierszeństwo operatorów w Pythonie:
1.Parentheses (grouping) ()
2. Function call f(args…)
3.Slicing e.g list, string x[index:index]
4. Subscription x[index]
5. Atribute reference. x.attribute
6. ** Exponentiation (potęga)
7 ~ NOT, also known as bitwise not - inverts all the bits ~x
8. Positive, negative +x -x
9. Multiplication, division, remainder
* / %
10. Addition, subtraction + -

11. Bitwise shifts e.g
    « Zero fill left shift Shift left by pushing zeros in from the right and let the leftmost bits fall off
    » Signed right shift Shift right by pushing copies of the leftmost bit in from the left, and let the rightmost bits fall off
12. Bitwise AND &
    Sets each bit to 1 if both bits are 1
13. Bitwise XOR ^
    Sets each bit to 1 if only one of two bits is 1
14. Bitwise OR |
    Sets each bit to 1 if one of two bits is 1
15. Comparision, membership identity
    in, not in, is, is not, <, <=, >, >=, <>, !=, ==
16. Boolean NOT not x
17. Boolean AND and
18. Boolean OR or
19. Lambda expression lambda

If operators have the same precedence then they will be evaluated from left to right, i.e addition will be applied first then subtraction
e.g
3 + 4 - 2
> 5

The only exception to this rule is assignment operator (=) which occur from right to left.
e.g
a = b = c

Question 40

Augmented Assignment Operators

Answer 40

+= , -=, = , /=, //=, %=, **=
These operators allows you write shortcut assignment statements. For e.g:
count = 1
count += 1
print(count)
> 2

Is the same as:
count = 1
count = count + 1
print(count)
> 2

Question 41

+=

Answer 41

Addition assignment
Example:
x += 4
Same as:
x = x + 4

Question 42

-=

Answer 42

Subtraction assignment
Example:
x -= 2
Same as:
x = x - 2

Question 43

*=

Answer 43

Multiplication assignment
Example:
x *= 5
Same as:
x = 5 * x

Question 44

/=

Answer 44

Division assignment
Example:
x /= 5
Same as:
x = x / 5

Question 45

//=

Answer 45

Integer division assignment
Example:
x //= 5
Same as:
x = x // 5

Question 46

//=

Answer 46

Integer division assignment
Example:
x //= 5
Same as:
x = x // 5

Question 47

%=

Answer 47

Remainder assignment
Example:
x %= 5
Same as:
x = x % 5

Question 48

**=

Answer 48

Exponent assignment
Example:
x **= 5
Same as:
x = x ** 5

Question 49

Strings

Answer 49

Strings in python are contiguous series of characters delimited by single or double quotes. Python doesn’t have any separate data type for characters so they are represented as a single character string.

Question 50

How to create strings?

Answer 50

name = “tom” # a string
mychar = ‘a’ # a character

You can also use the following syntax to create strings.

name1 = str() # this will create empty string object
name2 = str(“newstring”) # string object containing ‘newstring’

Question 51

id()

Answer 51

Every object in python is stored somewhere in memory. We can use id() to get that memory address.
e.g
str1 = “welcome”
str2 = “welcome”

id(str1)
>78965411

id(str2)
>78965411
As both str1 and str2 points to the same memory location, hence they both point to the same object.

Question 52

Are strings mutable?

Answer 52

Strings in Python are Immutable.
What this means to you is that once a string is created it can’t be modified. Let’s take an example to illustrate this point.
e.g
str1 = “welcome”
str2 = “welcome”
id(str1)
>78965411
id(str2)
>78965411

Let’s try to modify str1 object by adding a new string to it.

str1 += “ mike”
print(str1)
>welcome mike
id(str1)
>78965579

As you can see now str1 points to totally different memory location, this proves the point that concatenation doesn’t modify the original string object instead it creates a new string object.

Question 53

Concatanation

Answer 53

String concatenation is the operation of joining character strings end-to-end.

For example, the concatenation of “snow” and “ball” is “snowball”

Question 54

Are int type mutable?

Answer 54

Number (i.e int type) is, like string, immutable.
e.g

int1 = 1
int2 = 1
print(id(int1), id(int2))
> 9316800, 9316800

int2 += 3
print(int2)
print(id(int1), id(int2))
> 4
9316800, 9316896

Question 55

Could I access string characters?

Answer 55

Yes.
String index starts from 0, so to access the first character in the string type:

name = “tom”

print(name[0])
print(name[1])
>t
o

Question 56

Operator for string contatanation

Answer 56

+

e.g
s = “tom and “ + “jerry”
print(s)
> tom and jerry

Question 57

Operator for string repetition

Answer 57

*
e.g
s = “spamming is bad “ * 3
print(s)
> ‘spamming is bad spamming is bad spamming is bad ‘

Question 58

Operator for string slicing

Answer 58

[]

e.g
s = “Welcome”
s[1:3]
> el

Cięcie następuje przed [1] i przed [3]

Some more slicing examples.
s = “Welcome”

s[:6]
> ‘Welcom’

s[4:]
>’ome’

s[1:-1]
>’elcom’

The start index and end index are optional. If omitted then the default value of start index is 0 and that of end is the last index of the string.

Question 59

ord()

Answer 59

function returns the ASCII code of the character

print(ord(‘a’))
> 97

Question 60

chr()

Answer 60

chr() - function returns character represented by a ASCII number.

print(chr(97))
> a

Question 61

Name 3 string function in Python

Answer 61

len() - returns length of the string
max() - returns character having highest ASCII value
min() - returns character having lowest ASCII value

e.g
print(len(“jagoda”))
print(max(“jagoda”))
print(min(“jagoda”))

<6
o
a

Question 62

ASCII

Answer 62

ASCII (czyt. aski, skrót od ang. American Standard Code for Information Interchange) – siedmiobitowy system kodowania znaków.

Jest używany we współczesnych komputerach oraz sieciach komputerowych, a także innych urządzeniach wyposażonych w mikroprocesor.

Przyporządkowuje liczbom z zakresu 0−127:
- litery alfabetu łacińskiego języka angielskiego,
- cyfry,
- znaki przestankowe

i inne symbole oraz polecenia sterujące.

Na przykład litera „a” jest kodowana jako liczba 97, a znak spacji jest kodowany jako 32.

Question 63

membership operator

Answer 63

in, not in operators
e.g
s1 = “Welcome”
“come” in s1
>True

“come” not in s1
>False
»>

Question 64

What is used to compare strings with each other?

Answer 64

Python compares string lexicographically i.e using ASCII value of the characters.

You can use ( > , < , <= , <= , == , != ) to compare two strings.

Suppose you have str1 as “Mary” and str2 as “Mac”. The first two characters from str1 and str2 ( M and M ) are compared. As they are equal, the second two characters are compared. Because they are also equal, the third two characters (r and c ) are compared. And because r has greater ASCII value than c, str1 is greater than str2.

e.g
print(“Mac” == “Mary”)
print(“Mac” < “Mary”)
print(ord(“c”)), print(ord(“r”))

> False
True
99, 114

Question 65

By default, print() function prints string with a newline, we change this behavior by passing named keyword argument called ____

Answer 65

end
print(“my string”, end=” “)
> my string

print(“my string”, end=”foo”)
>my stringfoo

print(“my string”)
print(“my string”, end=”foo”)
> my string
my stringfoo

print(“my string”, end=””)
print(“my string”, end=”foo”)
> my stringmy stringfoo

Question 66

Can I use for loop on strings?

Answer 66

String is a sequence type and also iterable using for loop

e.g
word = “hello”
for letter in word:
print(letter)
>h
e
l
l
o

Print na końcu automatycznie dodaje /n. Możemy go jednak zastąpić pustym stringiem

e.g
word = “hello”
for letter in word:
print(letter, end=””)
> hello

Question 67

Testing strings: isalnum()

Answer 67

Returns True if string is alphanumeric

e.g
s = “welcome to python”
s.isalnum()
> False (bo jest spacja)

txt = “Company12”
x = txt.isalnum()
print(x)
>True

txt = “Company”
x = txt.isalnum()
print(x)
> True

txt = “12”
x = txt.isalnum()
print(x)
>True

Question 68

Testing strings: isalpha()

Answer 68

Returns True if string contains only alphabets

e.g
s = “welcome”
s.isalpha()
> True

s = “welcome to python”
s.isalpha()
> False

Question 69

Testing strings: isdigit()

Answer 69

Returns True if string contains only digits

e.g
“2012”.isdigit()
>True

Question 70

Testing strings: isidentifier()

Answer 70

Return True is string is valid identifier

A string is considered a valid identifier if it only contains alphanumeric letters (a-z) and (0-9), or underscores (_). A valid identifier cannot start with a number, or contain any spaces

Question 71

Testing strings: islower()

Answer 71

Returns True if string is in lowercase

e.g
s = “welcome’’
s.islower()
>True

Question 72

Testing strings: isupper()

Answer 72

Returns True if string is in uppercase

e.g
“WELCOME”.isupper()
>True

Question 73

Testing strings: isspace()

Answer 73

Returns True if string contains only whitespace

e.g
“ \t”.isspace()
> True

Question 74

Searching for Substrings: endswith(s1: str):
bool

Answer 74

Returns True if strings ends with substring s1

e.g
s = “welcome to python”
print(s.endswith(“thon”))
> True

Question 75

Searching for Substrings:
startswith(s1: str):
bool

Answer 75

Returns True if strings starts with substring s1

e.g
s = “welcome to python”
print(s.startswith(“good”))
>False

Question 76

Searching for Substrings:
count(substring):
int

Answer 76

Returns number of occurrences of substring in the string

e.g
s = “welcome to python”
print(s.count(“o”))
>3

Question 77

Searching for Substrings:
find(s1):
int

Answer 77

Returns lowest index from where s1 starts in the string, if string not found returns -1
(znajduje indeks od którego zaczyna się substring o któy pytamy, jeśli nic nie znajdzie drukuje -1)

e.g
s = “welcome to python”
print(s.find(“come”))
> 3
print(s.find(“become”))
> -1

Question 78

Searching for Substrings:
rfind(s1):
int

Answer 78

Returns highest index from where s1 starts in the string, if string not found returns -1
(znajduje ostatni index w którym występuje litera lub pierwszy indeks w którym wystepuje dany substring)

e.g
s = “welcometopython”
print(s.rfind(“o”))
print(s.rfind(“top”))

> 13
7

Question 79

Converting Strings:
capitalize():
str

Answer 79

Returns a copy of this string with only the first character capitalized.

e.g
s = “string in python”
s1 = s.capitalize()
print(s1)
> String in python

Question 80

Converting Strings:
lower():
str

Answer 80

Return string by converting every character to lowercase

e.g
s = “This Is Test”
s1 = s.lower()
print(s1)
> this is test

Question 81

Converting Strings:
upper():
str

Answer 81

Return string by converting every character to uppercase

e.g
s = “This Is Test”
s1 = s.upper()
print(s1)
> THIS IS TEST

Question 82

Converting Strings:
title():
str

Answer 82

This function return string by capitalizing first letter of every word in the string

e.g
s = “this is test”
s1 = s.title()
print(s1)
> This Is Test

Question 83

Converting Strings:
swapcase():
str

Answer 83

Return a string in which the lowercase letter is converted to uppercase and uppercase to lowercase

e.g
s = “jAgOdA”
s1 = s.swapcase()
print(s1)

> JaGoDa

Question 84

Converting Strings:
replace(old\, new):
str

Answer 84

This function returns new string by replacing the occurrence of old string with new string

e.g
s = “Jagoda”
s1 = s.replace(“a”, “e”)
print(s1)

> Jegode

Question 85

How to create a list in Python?

Answer 85

list1 = [1, 2, 3]
other way list1 = list([1,2,3])

empty list:
list1 = []
list1 = list()

Question 86

Are lists mutable?

Answer 86

Yes, lists are mutable.

Question 87

How to acces list elements?

Answer 87

list[1]
access second element in the list

Question 88

Common List Operations:
x in s

Answer 88

True if element x is in sequence s, False otherwise

Question 89

Common List Operations:
x not in s

Answer 89

True if element x is not in sequence s, False otherwise

Question 90

Common List Operations:
s1 + s2

Answer 90

Concatenates two sequences s1 and s2

Question 91

Common List Operations:
n * s
s * n

Answer 91

n copies of sequence s concatenated

Question 92

Common List Operations:
s [i]

Answer 92

access ith element in sequence s.

Question 93

Common List Operations:
len(s)

Answer 93

Length (number of elements) of list

Question 94

Common List Operations:
min(s)

Answer 94

Smallest element in sequence s.

Question 95

Common List Operations:
max(s)

Answer 95

Largest element in sequence s.

Question 96

Common List Operations:
sum(s)

Answer 96

Sum of all numbers in sequence s.

Question 97

Common List Operations:
for loop

Answer 97

Przechodzi elementy od lewej do prawej w pętli for.

Question 98

Slice operator ([start:end])

Answer 98

Slice operator ([start:end]) allows to fetch sublist from the list. It works similar to string.

e.g
list = [11,33,44,66,788,1]

list[0:5] # this will return list starting from index 0 to index 4
>[11,33,44,66,788]

list[:3]
>[11,33,44]

list[2:]
>[44,66,788,1]

Question 99

The ___ operator joins the two list.

Answer 99

+

Question 100

The ___ operator replicates the elements in the list.

Answer 100

*

Question 101

The __ operator is used to determine whether the elements exists in the list. On success it returns True on failure it returns False.

Answer 101

in

Question 102

List is a sequence and also iterable - you can use ___ to traverse through all the elements of the list.

Answer 102

for i in

e.g
list = [1,2,3,4,5]

for i in list:
print(i, end=” “)
1 2 3 4 5

Question 103

Common list methods:
append

Answer 103

append(x:object):None

Adds an element x to the end of the list and returns None.

e.g
list1 = [2, 3, 4, 1, 32, 4]
list1.append(19)
print(list1)
>[2, 3, 4, 1, 32, 4, 19]

Question 104

Common list methods:
count

Answer 104

count(x:object):int
Returns the number of times element x appears in the list.

e.g
list1 = [2, 3, 4, 1, 32, 4]

list1.count(4) # Return the count for number 4
>2

Question 105

Common list methods:
extend

Answer 105

extend(l:list):None

Appends all the elements in l to the list and returns None.

e.g
list1 = [2, 3, 4, 1, 32, 4]
list2 = [99, 54]

list1.extend(list2)
print(list1)
> [2, 3, 4, 1, 32, 4, 19, 99, 54]

Question 106

Common list methods:
index

Answer 106

index(x: object):int
Returns the index of the first occurrence of element x in the list

e.g
list1 = [2, 3, 4, 1, 32, 4]
list1.index(4) # Return the index of number 4
> 2

Question 107

Common list methods:
insert

Answer 107

insert(index: int, x: object):None

Inserts an element x at a given index. Note that the first element in the list has index 0 and returns None.

e.g
list1 = [2, 3, 4, 1, 32, 4]
list1.insert(1, 25) # Insert 25 at position index 1
print(list1)
> [2, 25, 3, 4, 1, 32, 4, 19, 99, 54]

Question 108

Common list methods:
remove

Answer 108

remove(x:object):None
Removes the first occurrence of element x from the list and returns None

e.g
list1 = [2, 3, 4, 1, 32, 4]
list1.remove(32) # Remove number 32
print(list1)
>[2, 25, 4, 1, 4, 19, 99]

Question 109

What ‘returns None’ mean?

Answer 109

that function ends succesfully

Question 110

Common list methods:
reverse

Answer 110

reverse():None
Reverse the list and returns None

e.g
list1 = [2, 3, 4, 1, 32, 4]
list1.reverse() # Reverse the list
print(list1)
>[4, 32, 1, 4, 3, 2]

Question 111

Common list methods:
sort

Answer 111

sort(): None
Sorts the elements in the list in ascending order and returns None.

e.g
list1 = [2, 3, 4, 1, 32, 4]
»> list1.sort() # Sort the list
print(list1)
>[1, 2, 4, 4, 19, 25, 99]

Question 112

Common list methods:
pop

Answer 112

pop(i): object
Removes the element at the given position and returns it. The parameter i is optional. If it is not specified, pop() removes and returns the last element in the list.

e.g
list1 = [2, 25, 4, 1, 32, 4, 19, 99, 54]
list1.pop()

print(list1)
[2, 25, 4, 1, 32, 4, 19, 99]

Question 113

List comprehension

Answer 113

List comprehension provides a concise way to create list. It consists of square brackets containing expression followed by for clause then zero or more for or if clauses.

> > > list1 = [ x for x in range(10) ]
list1
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]list2 = [ x + 1 for x in range(10) ]
list2
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]list3 = [ x for x in range(10) if x % 2 == 0 ]
list3
[0, 2, 4, 6, 8]list4 = [ x *2 for x in range(10) if x % 2 == 0 ]
[0, 4, 8, 12, 16]

Question 114

Dictionary

Answer 114

Dictionary is a python data type that is used to store key-value pairs. It enables you to quickly retrieve, add, remove, modify, values using a key. Dictionary is very similar to what we call associative array or hash on other languages.

e.g
friends = {
‘tom’ : ‘111-222-333’,
‘jerry’ : ‘666-33-111’
}

here friends is a dictionary with two items. One point to note that key must be of a hashable type, but the value can be of any type. Each key in the dictionary must be unique.

Question 115

How to create mepty dictionary?

Answer 115

> > > dict_emp = {} # this will create an empty dictionary

Alternatively:
dict_eml = dict()

Question 116

How to get an item from the dictionary?

Answer 116

dictionary_name[‘key’]

e.g
friends = {
‘tom’ : ‘111-222-333’,
‘jerry’ : ‘666-33-111’
}

friends[‘tom’]
> ‘111-222-333’

If the key exists in the dictionary, the value will be returned, otherwise a KeyError exception will be thrown.

Question 117

How to delete items from the dictionary?

Answer 117

dictionary_name[‘newkey’] = ‘newvalue’

e.g
friends = {
‘tom’ : ‘111-222-333’,
‘jerry’ : ‘666-33-111’
}

del friends[‘bob’]
friends
> {‘tom’: ‘111-222-333’, ‘jerry’: ‘666-33-111’}

If the key is found the item will be deleted, otherwise a KeyError exception will be thrown.

Question 118

How to use for loop to traverse elements in the dictionary?

Answer 118

friends = {
‘tom’ : ‘111-222-333’,
‘jerry’ : ‘666-33-111’
}

for key in friends:
print(key, “:”, friends[key])

> tom : 111-222-333
jerry : 666-33-111

Question 119

How to find the length of the dictionary?

Answer 119

You can use the len() function to find the length of the dictionary.

friends = {
‘tom’ : ‘111-222-333’,
‘jerry’ : ‘666-33-111’
}

len(friends)
>2

Question 120

How to check whether key exists in the dictionary?

Answer 120

in and not in operators to check whether key exists in the dictionary.

friends = {
‘tom’ : ‘111-222-333’,
‘jerry’ : ‘666-33-111’
}

‘tom’ in friends
> True
‘tom’ not in friends
> False

Question 121

How to test whether two dictionaries contains the same items ?

Answer 121

The == and != operators tells whether dictionary contains the same items or not.

d1 = {“mike”:41, “bob”:3}
d2 = {“bob”:3, “mike”:41}
d1 == d2
> True
d1 != d2
> False

Question 122

Dictionary methods:
popitem()

Answer 122

Returns randomly selected item from the dictionary and also remove the selected item.

e.g
friends = {
‘tom’: ‘111-222-333’,
‘bob’: ‘888-999-666’,
‘jerry’: ‘666-33-111’
}

friends.popitem()
>(‘tom’, ‘111-222-333’)

Question 123

Dictionary methods:
clear()

Answer 123

Delete everything from a dictionary

e.g
friends = {
‘tom’: ‘111-222-333’,
‘bob’: ‘888-999-666’,
‘jerry’: ‘666-33-111’
}

friends.clear()
friends
> {}

Question 124

Dictionary methods:
keys()

Answer 124

Return keys in the dictionary as tuples

e.g
friends = {
‘tom’: ‘111-222-333’,
‘bob’: ‘888-999-666’,
‘jerry’: ‘666-33-111’
}

friends.keys()
> dict_keys([‘tom’, ‘bob’, ‘jerry’])

Question 125

Dictionary methods:
get(key)

Answer 125

Return value of key, if key is not found it returns None, instead of throwing KeyError exception

e.g
friends = {
‘tom’: ‘111-222-333’,
‘bob’: ‘888-999-666’,
‘jerry’: ‘666-33-111’
}

friends.get(‘tom’)
>’111-222-333’

Question 126

Dictionary methods:
pop(key)

Answer 126

Remove the item from the dictionary, if the key is not found KeyError will be thrown

e.g
friends = {
‘tom’: ‘111-222-333’,
‘bob’: ‘888-999-666’,
‘jerry’: ‘666-33-111’
}

friends.pop(‘bob’)
friends
> {‘tom’: ‘111-222-333’, ‘jerry’: ‘666-33-111’}

Question 127

Relational operators

Answer 127

Relational operators allows us to compare two objects.

Question 128

Relational operators
<=

Answer 128

smaller than or equal to

Question 129

Relational operators
<=

Answer 129

smaller than or equal to

Question 130

Relational operators
<, >

Answer 130

smaller than
greater than

Question 131

Relational operators
>=

Answer 131

greater than or equal to

Question 132

Relational operators

Answer 132

equal to

Question 133

Relational operators
!=

Answer 133

not equal to

Question 134

Example of statement with boolean-expression

Answer 134

if boolean-expression:
#statements
else:
#statements

e.g
i = 10

if i % 2 == 0:
print(“Number is even”)
else:
print(“Number is odd”)

Question 135

Example of nested if statement

Answer 135

today = “holiday”
bank_balance = 25000
if today == “holiday”:
if bank_balance > 20000:
print(“Go for shopping”)
else:
print(“Watch TV”)
else:
print(“normal working day)

Question 136

Syntax for def

Answer 136

arg = argument/parameter

def function_name(arg1, arg2, arg3, …. argN):
#statement inside function

e.g
def myfunc():
pass

Example of a function that calculates the sum of all the numbers starting from start to end:

def sum(start, end)
result=0
for i in range(start, end+1)
result = result +1

s = sum(10, 50)
print(s)

> 1230

Question 137

In python, if you do not explicitly return value from a function, then a special value of _____ is always returned.

Answer 137

None

e.g
def test(): # test function with only one statement
i = 100
print(test()

> None

As you can see, the test() function doesn’t explicitly return any value, so None is returned.

Question 138

Global variables

Answer 138

Global variables: Variables that are not bound to any function , but can be accessed inside as well as outside the function are called global variables.

e.g
global_var = 12

def func():
local_var = 100
print(global_var)

you can access global variables inside function

Question 139

Local variables

Answer 139

Local variables: Variables which are declared inside a function are called local variables.

e.g
xy = 100

def cool():
xy = 200
print(xy)

cool()
> 200
print(xy)
> 100

W przypadku gdy mamy globalną i lokalna zmienną o takiej samej nazwie i wywołujemy funkcję zawierającą tę lokalną zmienną, to lokalna zmienna bedzie miała pierwszeństwo i nie bedziemy mogli wywołać globalnej zmiennej o tej samej nazwie wewnątrz tej funkcji.

Question 140

How to convert local variable into global variable?

Answer 140

You can bind local variable in the global scope by using the global keyword followed by the name of variable

e.g
t = 1

def increment():
global t
t = t + 1
print(t)

# now t inside the function is same as t outside the function

In fact there is no need to declare global variables outside the function. You can declare them global inside the function.

e.g

def foo():
global x
x = 100

Question 141

How to specify default values of argument?

Answer 141

To specify default values of argument, you just need to assign a value using assignment operator.

e.g
def func(i, j = 100):
print(i, j)

Above function has two parameter i and j. The parameter j has a default value of 100, it means that we can omit value of j while calling the function.
i and j are positional argument

Question 142

Keyword arguments

Answer 142

Keyword arguments allows you to pass each arguments using name value pairs like this name=value. Let’s take an example:

e.g

def named_args(name, greeting):
print(greeting + “ “ + name )

named_args(name=’jim’, greeting=’Hello’)

> Hello jim

Question 143

Example of mixing positional and keyword arguments

Answer 143

def my_func(a, b, c):
print(a, b, c)

my_func(12, 13, 14)
> 12, 13, 14

my_func(12, c=14, b=13)
> 12, 13, 14

Question 144

How to return multiple values from the function?

Answer 144

We can return multiple values from function using the return statement by separating them with a comma (,). Multiple values are returned as tuples.

e.g

def bigger(a, b):
if a > b:
return a, b
else:
return b, a

Question 145

How many loop types of loop Python has?

Answer 145

Python has only two loops:

for loop
while loop

All the statements inside for and while loop must be indented to the same number of spaces. Otherwise, SyntaxError will be thrown.

Question 146

The for loop Syntax:

Answer 146

for i in iterable_object:
# do something

e.g.
my_list = [1,2,3,4]

for i in my_list:
print(i)

> 1
2
3
4

Question 147

range(a, b)

Answer 147

for i in range(1, 10):
print(i)

> 1
2
3
4
5
6
7
8
9

We receive the same output, when we type:

for i in range(10):
… print(i)

> 0
1
2
3
4
5
6
7
8
9

Question 148

range(a, b, c)

Answer 148

The range(a, b) function has an optional third parameter to specify the step size.

e.g

for i in range(1, 20, 2):
print(i)

> 1
3
5
7
9
11
13
15
17
19

Question 149

The while loop Syntax:

Answer 149

while condition:
# do something

e.g
count = 0

while count < 10:
print(count)
count += 1

> 0
1
2
3
4
5
6
7
8
9

Question 150

The while loop Syntax:

Answer 150

while condition:
# do something

e.g
count = 0

while count < 10:
print(count)
count += 1

> 0
1
2
3
4
5
6
7
8
9

Question 151

break statement

Answer 151

The break statement allows to breakout out of the loop.

e.g
count = 0

while count < 10:
count += 1
if count == 5:
break
print(“inside loop”, count)
print(“out of while loop”)

> inside loop 1
inside loop 2
inside loop 3
inside loop 4
out of while loop

Question 152

continue statement

Answer 152

When continue statement encountered in the loop, it ends the current iteration and programs control goes to the end of the loop body.

e.g
count = 0

while count < 10:
count += 1
if count % 2 == 0:
continue
print(count)

> 1
3
5
7
9

Question 153

Python Mathematical Function:
round(number[, ndigits])

Answer 153

rounds the number, you can also specify precision in the second argument

e.g
round(62.998, 1)
> 63.0

Question 154

Python Mathematical Function:
pow(a, b)

Answer 154

Returns a raise to the power of b

e.g
pow(3, 2)

> 9

Question 155

Python Mathematical Function:
abs(x)

Answer 155

Return absolute value of x

e.g
abs(-22)
>22

Question 156

Python Mathematical Function:
max(x1, x2, …, xn)

Answer 156

Returns largest value among supplied arguments

e.g
max(9, 3, 12, 81)
>81

Question 157

Python Mathematical Function:
min(x1, x2, …, xn)

Answer 157

Returns smallest value among supplied arguments

e.g
min(78, 99, 12, 32)
>12

Question 158

Python Mathematical Function:
import math
ceil(x)

Answer 158

This function rounds the number up and returns its nearest integer

math.ceil(3.4123)
> 4

Question 159

Python Mathematical Function:
import math
floor(x)

Answer 159

This function rounds the down up and returns its nearest integer

math.floor(24.99231)
>24

Question 160

Python Mathematical Function:
import math
sqrt(x)

Answer 160

Returns the square root of the number

e.g
math.sqrt(16)
> 4.0

Question 161

Python Mathematical Function:
import math
sin(x)

Answer 161

Returns sin of x where x is in radian

e.g
math.sin(1)
> 8414709848078965

Question 162

Python Mathematical Function:
import math
cos(x)

Answer 162

Returns cosine of x where x is in radian

e.g
math.cos(2))
> -0.4161468365471424

Question 163

Python Mathematical Function:
import math
tan(x)

Answer 163

Returns tangens of x where x is in radian

e.g
math.tan(9)
> -0.4523156594418099

Question 164

Python ________ module contains function to generate random numbers.

Answer 164

random

You need to import random module using the following line:
import random

Question 165

import random

random()

Answer 165

returns random number between 0 and 1

e.g
for i in range(0, 3):
print(random.random())

> 0.5994361280943458
0.023307374459719976
0.7901690676332683

Question 166

import random

randint()

Answer 166

randint(a, b)

generate random numbers between a and b (inclusively)

e.g
for i in range(0, 3):
print(random.randint(1, 20))

> 5
8
18

Question 167

Syntax for opening the file in Python

Answer 167

We need to use open() function

open(filename, mode)

The open() function accepts two arguments filename and mode. The filename is a string argument which specify filename along with it’s path.

Mode is a string argument which is used to specify how file will be used i.e for reading or writing.

We usually use open() function assigning it to f known as file pointer.

Syntax of opening a file is:
f = open(filename, mode)

Question 168

Which method is used to close the file

Answer 168

close()

f.close() # where f is a file pointer

Question 169

Modes of opening a file:
“r”

Answer 169

Open a file for read only

Question 170

Modes of opening a file:
“w”

Answer 170

Open a file for writing. If file already exists its data will be cleared before opening. Otherwise new file will be created

# open file for writing
f = open(‘myfile.txt’, ‘w’)

# write a line to the file
»> f.write(‘this first line\n’) *

# write one more line to the file
»> f.write(‘this second line\n’)

close the file
»> f.close()

*The write() method will not insert new line (‘\n’) automatically like print() function, you need to explicitly add ‘\n’ to write write() method.

Question 171

Modes of opening a file:
“a”

Answer 171

Opens a file in append mode i.e to write a data to the end of the file

Question 172

Modes of opening a file:
“wb”

Answer 172

Open a file to write in binary mode

Question 173

Modes of opening a file:
“rb”

Answer 173

Open a file to read in binary mode

Question 174

Reading data from the file in Python
read([number])

Answer 174

Return specified number of characters from the file. if omitted it will read the entire contents of the file.

Question 175

Reading data from the file in Python
readline()

Answer 175

Return the specified line of the file.

Question 176

Reading data from the file in Python
readlines()

Answer 176

Read all the lines as a list of strings in the file

Question 177

Reading data from the file in Python

Answer 177

read()
Reading all the data at once.

Question 178

Reading data from the file in Python

Answer 178

Reading all the data at once.

open(“file.txt”, “r”) as f:
print (f.read())

Question 179

To append the data to a file, you need to ________

Answer 179

open the file in ‘a’ mode.

e.g

f = open(‘myfile.txt’, ‘a’)
f.write(“this is third line\n”)
>19
f.close()

Question 180

How to use for loop on a file?

Answer 180

f = open(‘myfile.txt’, ‘r’)
for line in f:
print(line)

> this first line
this second line
this is third line

f.close()

Question 181

To perform binary reading and writing you need to use a module called _______

Answer 181

pickle

Question 182

import pickle

What is the method to write binary data?

Answer 182

dump

e.g
import pickle
f = open(‘pick.dat’, ‘wb’)

pickle.dump(11, f)
pickle.dump(“this is a line”, f)
pickle.dump([1, 2, 3, 4], f)

f.close()

Question 183

import pickle

What is the method to read binary data?

Answer 183

load

import pickle

f = open(‘pick.dat’, ‘rb’)
pickle.load(f)
> 11

pickle.load(f)
> “this is a line”
»> pickle.load(f)
[1,2,3,4]

f.close()

If there is no more data to read from the file, pickle.load() throws EOFError or end of file error.

Question 184

What is an object in Python?

Answer 184

In python everything is object i.e int, str, bool even modules, functions are also objects.

Python is an object-oriented language

Question 185

How to define class in Python?

Answer 185

Class name in python is preceded with class keyword followed by a colon (:).

e.g
class Person:
def __init__(self, name):
self.name = name

 def whoami(self):
       return "You are " + self.name

Question 186

What are classes for?

Answer 186

Classes commonly contains data field to store the data and methods for defining behaviors.

Question 187

What is always the first def in class?

Answer 187

A special method called initializer (commonly known as constructor).

It is invoked automatically every time new object is created.

e.g
class Person:

   # constructor or initializer
  def \_\_init\_\_(self, name): 
        self.name = name 

 def whoami(self):
       return "You are " + self.name

Question 188

Each class has _____

Answer 188

instance variables

e.g
class Person:
def __init__(self, name):
self.name = name*

• name is data field also commonly known as instance variables

Question 189

Create a class called Person which contains one data field called name and method whoami().

Answer 189

class Person:
def __init__ (self, name):
self.name = name
def whoami (self):
print(f”You are {self.name}.”)

name = Person(“Jagoda”)
print(name.whoami())

> You are Jagoda.

Question 190

What is self?

Answer 190

All methods in python including some special methods like initializer have first parameter self. This parameter refers to the object which invokes the method. When you create new object the self parameter in the __init__ method is automatically set to reference the object you have just created.

Question 191

How to create private data fields in Python?

Answer 191

To create private (hide) datafields in Python in a way that they are not accessible outside the class, we need to use two underscores

e.g

class BankAccount:

 # constructor or initializer
def \_\_init\_\_(self, name, money):
     self.\_\_name = name
     self.\_\_balance = money   # \_\_balance is private now, so it is only accessible inside the class

def deposit(self, money):
     self.\_\_balance += money

def withdraw(self, money):
     if self.\_\_balance > money :
         self.\_\_balance -= money
         return money
     else:
         return "Insufficient funds"

def checkbalance(self):
     return self.\_\_balance

b1 = BankAccount(‘tim’, 400)
print(b1.withdraw(500))
> Insufficient funds

b1.deposit(500)
print(b1.checkbalance())
> 900

print(b1.withdraw(800))
> 800

print(b1.checkbalance()
> 100

Let’s try to access __balance data field outside of class.

print(b1.__balance)
> AttributeError: ‘BankAccount’ object has no attribute ‘__balance’

Question 192

+ operator is overloaded by both __ class and __class.

Answer 192

int, str

Question 193

What is operator overloading?

Answer 193

Defining methods for operators

e.g To use + operator with custom objects you need to define a method called __add__.

Question 194

_ _add__ is used for …

Answer 194

using + operators with custom objects

e.g
class Circle:

def \_\_init\_\_(self, radius):
    self.\_\_radius = radius

def setRadius(self, radius):
    self.\_\_radius = radius

def getRadius(self):
    return self.\_\_radius

def \_\_add\_\_(self, another_circle):
    return Circle( self.\_\_radius + another_circle.\_\_radius )

c1 = Circle(4)
print(c1.getRadius())
> 4

c2 = Circle(5)
print(c2.getRadius())
> 5

c3 = c1 + c2 # This became possible because we have overloaded + operator by adding a method named __add__

print(c3.getRadius())
> 9

In the above example we have added __add__() method which allows use to use the + operator to add two circle objects. Inside the __add__() method we are creating a new object and returning it to the caller.
Python has many other special methods like __add__().

Question 195

Operator Overloading
+

Answer 195

Addition:
__add__(self, other)

Question 196

Operator Overloading
*

Answer 196

Multiplication:
__mul__(self, other)

Answer 197

Subtraction:
__sub__(self, other)

Question 198

Operator Overloading
%

Answer 198

modulus (remainder):
__mod__(self, other)

bo % to modulo operator
modulo operator ( % ), returns the remainder of dividing two numbers.

Question 199

Operator Overloading
/

Answer 199

Division:
__truediv__(self, other)

Question 200

Operator Overloading
<

Answer 200

Less than:
__lt__(self, other)

Question 201

Operator Overloading
<=

Answer 201

Less than or equal to:
__le__(self, other)

Question 202

Operator Overloading

Answer 202

Equal to:
__eq__(self, other)

Question 203

Operator Overloading
!=

Answer 203

Not equal to:
__ne__(self, other)

Question 204

Operator Overloading
>

Answer 204

Greater than:
__gt__(self, other)

Question 205

% in Python means:

Answer 205

remainder

Question 206

Inheritance:
In object-oriented terminology when class B extend class A, then A is called _____ class and B is called ___ class.

Answer 206

A is called super class or base class
B is called subclass, derived class or child class.

Question 207

Inheritance:
What kinds of methods and data fields are accessible by child class? Which are not?

Answer 207

Not private methods and data fields are accessible by child class.
private data fields and methods are accessible only inside the class.

Question 208

Example of class inheritance

Answer 208

class Vehicle:

def \_\_init\_\_(self, name, color):
    self.\_\_name = _name     
    self.\_\_color = color   # \_\_name is private to Vehicle class

def getColor(self):         
    return self.\_\_color # getColor() function is accessible to class Car

def setColor(self, color): 
    self.\_\_color = color # setColor is accessible outside the class

def getName(self):              
     return self.\_\_name # getName() is accessible outside the class

class Car(Vehicle):

def \_\_init\_\_(self, name, color, model):
    super().\_\_init\_\_(name, color)       
    self.\_\_model = model
    # call parent constructor to set name and color  

def getDescription(self):
    return self.getName() + self.\_\_model + " in " +  \n
    self.getColor() + " color"

in method getDescrition we are able to call getName(), getColor() because they are
# accessible to child class through inheritance

Question 209

_____ method is used to call method of the base class.

Answer 209

super()

Suppose you need to call a method called get_information() in the base class from child class , you can do so using the following code.
super().get_information()

Similarly, you can call base class constructor from child class constructor using the following code.
super().__init__()

Question 210

What is multiple inheritance? Is it possible in Python?

Answer 210

Multiple inheritance means that you can create class, which inherit from multiple classes at the same time.
It is not possible in Java and C programming languages

class Subclass(SuperClass1, SuperClass2, …):
# initializer
# methods

e.g
class MySuperClass1():

def method_super1(self):
    print("method_super1 method called")

class MySuperClass2():

def method_super2(self):
    print("method_super2 method called")

class ChildClass(MySuperClass1, MySuperClass2):

def child_method(self):
    print("child method")

c = ChildClass()
c.method_super1()
c.method_super2()

Question 211

Overriding methods

Answer 211

To override a method in the base class, sub class needs to define a method of same signature. (i.e same method name and same number of parameters as method in base class).

(metody child class nadpisują metody klasy bazowej)

Question 212

isinstance() function

Answer 212

The isinstance() function is used to determine whether the object is an instance of the class or not.

Question 213

What is polymorphism in python?

Answer 213

Polymorphism is the ability of an object to take on many forms.
The most common use of polymorphism in OOP occurs when a parent class reference is used to refer to a child class object.

Question 214

What is used to handle exception?

Answer 214

Python handles exception using try, except block.

try:
# write some code
# that might throw exception
except <ExceptionType>:
# Exception handler, alert the user</ExceptionType>

e.g
try:
f = open(‘somefile.txt’, ‘r’)
print(f.read())
f.close()
except IOError:
print(‘file not found’)

A try statement can have more than once except clause, It can also have optional else and/or finally statement.

try:
<body>
except <ExceptionType1>:
<handler1>
except <ExceptionTypeN>:
<handlerN>
except:
<handlerExcept>
else:
<process_else>
finally:
<process_finally></process_finally></process_else></handlerExcept></handlerN></ExceptionTypeN></handler1></ExceptionType1>

Statements under the else clause run only when no exception is raised.

Statements in the finally clause will run every time no matter exception occurs or not.

Question 215

eval()

Answer 215

The eval() function evaluates the specified expression, if the expression is a legal Python statement, it will be executed.
e.g Evaluate the expression ‘print(55)’:
x = ‘print(55)’
eval(x)

Question 216

raise

Answer 216

To raise your exceptions from your own methods you need to use raise keyword like this

def enterage(age):
if age < 0:
raise ValueError(“Only positive integers are allowed”)

Question 217

How to assign exception object to a variable?

Answer 217

try:
# this code is expected to throw exception
except ExceptionType as ex:
# code to handle exception

e.g
try:
number = eval(input(“Enter a number: “))
except NameError as ex:
print(“Exception:”, ex)
>Exception: name ‘one’ is not defined

Question 218

As you can see from most of the exception classes in python extends from the _______ class.

Answer 218

BaseException

You can derive you own exception class from BaseException class or from some sublcass of BaseException like RuntimeError (depends on what you need)

e.g
class NegativeAgeException(RuntimeError):
def __init__(self, age):
super().__init__()
self.age = age

Above code creates a new exception class named NegativeAgeException, which consists of only constructor which call parent class constructor using super().__init__() and sets the age.

Question 219

What is Python module?

Answer 219

Python module is a normal python file which can store function, variable, classes, constants etc. Module helps us to organize related codes . For e.g math module in python has mathematical related functions.

To create a new module, just save .py file in your project folder.

Question 220

To use module in another file, we need to….

Answer 220

import mymodule

to use module in our programs we first need to import it using import statement

Question 221

When you want to access only specific function or variable from another module you use ___________ instead of import

Answer 221

from mymodule import myfunction

Question 222

dir()

Answer 222

Comparing Python and Node.Js: Which Is Best for Your Project?

The dir() returns a list of string containing the names of the available attributes.

e.g

dir(mymodule)

> [‘__builtins__’, ‘__cached__’, ‘__doc__’, ‘__file__’,
‘__loader__’, ‘__name__’, ‘__package__’, ‘__spec__’, ‘foo’, ‘hello’]

Question 223

Regular Expression

Answer 223

Regular expression is widely used for pattern matching.

Python has built-in support for regular function. To use regular expression you need to import re module.

Question 224

import re
re.search()

Answer 224

The re.search() is used to find the first match for the pattern in the string.

Syntax:
re.search(pattern, string, flags[optional])

e.g
import re

example= “my is 123”
print(re.search(r”\d\d\d”, example))
> <re.Match object; span=(6, 9), match=’123’>

r to przedrostek, którym poprzedzamy pattern lub string, który wyszukujemy
/d/d/d to patern na wyszukanie 3 liczb (d od digits)

UWAGA:
re.search() find only first match for the pattern, if we want to find all matches in string, this is where findall() comes into the play.

Question 225

Basic patterns used in regular expression

Answer 225

dot matches any character except newline
.

Question 226

Basic patterns used in regular expression
\w

Answer 226

word character

matches any word character i.e letters, alphanumeric, digits and underscore (_)

Question 227

Basic patterns used in regular expression
\W

Answer 227

non-word character

matches non word characters

Question 228

Basic patterns used in regular expression
\d

Answer 228

matches a single digit

Question 229

Basic patterns used in regular expression
\D

Answer 229

matches a single character that is not a digit

Question 230

Basic patterns used in regular expression
\s

Answer 230

matches any white-spaces character like \n, \t, spaces

e.g
import re

example= “my is 123”
print(re.search(r”\w\w\s”, example))
><re.Match object; span=(0, 3), match=’my ‘>

Question 231

Basic patterns used in regular expression
\S

Answer 231

matches single non white space character

e.g.

import re
example= “my is 123”
print(re.search(r”\s\S\S\s”, example))

> <re.Match object; span=(2, 6), match=’ is ‘>

Question 232

Basic patterns used in regular expression
[abc]

Answer 232

matches single character in the set i.e either match a, b or c

e.g
import re
example= “my is 123”
print(re.search(r”[mjx]”, example))
><re.Match object; span=(0, 1), match=’m’>

Question 233

Basic patterns
[a-z]

Answer 233

match a single character in the range a to z.

Question 234

Basic patterns used in regular expression
[^abc]

Answer 234

match a single character other than a, b and c

Question 235

Basic patterns used in regular expression
[a-zA-Z]

Answer 235

match a single character in the range a-z or A-Z

Question 236

Basic patterns used in regular expression
[0-9]

Answer 236

match a single character in the range 0-9

Question 237

Basic patterns used in regular expression

Answer 237

match start at beginning of the string

e.g
import re
example= “my is 123”
print(re.search(r”^”, example))

> <re.Match object; span=(0, 0), match=’’>

Question 238

Basic patterns used in regular expression
$

Answer 238

match start at end of the string

e.g
import re

example= “my is 123”
print(re.search(r”$”, example))

> <re.Match object; span=(9, 9), match=’’>

Question 239

Basic patterns used in regular expression
+

Answer 239

matches one or more of the preceding character (greedy match).

Question 240

Basic patterns used in regular expression
*

Answer 240

matches zero or more of the preceding character (greedy match).

Question 241

Regular expressions: jak wyszukać adres e-mail używając patternów?

Answer 241

import re
s = “tim email is tim@somehost.com”
match = re.search(r’[\w.-]+@[\w.-]+’, s)

the above regular expression will match a email address

if match:
print(match.group())
else:
print(“match not found”)

> Here we have used [\w.-]+@[\w.-]+ pattern to match an email address

Question 242

Regular expressions: Jak wyciągnąć imię i rodzaj poczty ze string

Answer 242

group capturing

Group capturing allows to extract parts from the matching string. You can create groups using parentheses ().

e.g
import re
s = “tim email is tim@somehost.com”
match = re.search(r’([\w.-]+)@([\w.-]+)’, s)

if match:
print(match.group()) ## tim@somehost.com (the whole match)
print(match.group(1)) ## tim (the username, group 1)
print(match.group(2)) ## somehost (the host, group 2)

> tim@somehost.com
tim
somehost.com

Question 243

re.findall()

Answer 243

is used instead of re.search(), when, we want to findall matches in string (re.search() stops after it finds first)

e.g
import re
s = “Tim’s phone numbers are 12345-41521 and 78963-85214”
match = re.findall(r’\d{5}’, s)

if match:
print(match)

> [‘12345’, ‘41521’, ‘78963’, ‘85214’]

Question 244

Can I use group capturing with re.findall()?

Answer 244

You can also use group capturing with findall(), when group capturing is applied then findall() returns a list of tuples where tuples will contain the matching groups

Question 245

What are flags in the following syntax for re.search() and re.findall():

re.search(pattern, string, flags[optional])

findall(pattern, string, flags=0[optional])

Answer 245

re.IGNORECASE, re.DOTALL and re.MULTILINE

re.IGNORECASE - ignores uppercase and lowercase

re.DOTALL - allows (.) to match newline, be default (.) matches any character except newline

re.MULTILINE - this will allow ^ and $ to match start and end of each lin

Question 246

Czym różni się re.search() od re.match()?

Answer 246

The re.match() is very similar to re.search() difference is that it will start looking for matches at the beginning of the string.

Question 247

What is *args?

Answer 247

The *args allows us to pass variable number of arguments to the function.

e.g
def sum(a, b):
print(“Result:”, a+b)
print(sum(2, 2) )
> 4

def sum(a, b):
print(“Result:”, a+b)
print(sum(2, 2, 3))
> Traceback (most recent call last): File… TypeError: sum() takes 2 positional arguments but 3 were given

But when we use *args:

def sum(*args):
s = 0
for i in args:
s += i
print(“Result:”, s)

print(sum(2, 2, 3))
> 7

Now you can pass any number of arguments to the function like this

Please note that, The name of *args is just a convention you can use anything that is a valid identifier. For e.g *myargs is perfectly valid.

Question 248

What is **kwargs?

Answer 248

**kwargs allows us to pass variable number of keyword argument like this (argumentów nazwanych):

func_name(name=’tim’, team=’school’)

e.g
def my_func(**kwargs):
for i, j in kwargs.items():
print(i, j)

print(my_func(name=’tim’, sport=’football’, roll=19))

Pozycyjne tu nie przechodzą

Question 249

You can use *args to pass elements in an iterable variable to a function.

Answer 249

def my_three(a, b, c):
print(a, b, c)

a = [1,2,3]
print(my_three(*a))
> 1 2 3

Podaliśmy tylko a, a dopasowało do a, b i c, *a to było *args

args = [1,2,3]
print(my_three(*args))
> 1 2 3

This works only when number of argument is same as number of elements in the iterable variable.

Similarly you can use **kwargs to call a function like this:

def my_three(a, b, c):
print(a, b, c)

a = {‘a’: “one”, ‘b’: “two”, ‘c’: “three” }
print(my_three(**a))
> one two three

Note that for this to work 2 things are necessary:

Names of arguments in function must match with the name of keys in dictionary.
Number of arguments should be same as number of keys in the dictionary.

Question 250

Python Generators

Answer 250

Generators are function used to create iterators, so that it can be used in the for loop.

Question 251

What is pip

Answer 251

PIP is a package management system used to install packages from repository. You can use pip to install various software packages available on http://pypi.python.org/pypi. PIP is much similar to composer in php. PIP is a recursive acronym which stands for PIP installs packages.

Question 252

How to install pip?

Answer 252

Python 3.4 and later (python 3 series) already comes with pip.
To check your python version you need to enter the following command :

python3 -V

Question 253

How to install, upgrade and uninstall packages?

Answer 253

pip3 install package_name

pip3 install –upgrade package_name

pip uninstall package_name

Question 254

How to search for packages?

Answer 254

pip search “your query”

but this command is no longer supported

Question 254

How to list installed packages?

Answer 254

pip3 list

Question 255

How to list outdated installed packages?

Answer 255

pip3 list –outdated

Question 256

How to get information about an installed package?

Answer 256

pip3 show package_name

Question 257

What is virtualenv?

Answer 257

virtualenv is a tool used to separate different dependencies required by the projects. While working on multiple projects it’s a common issue that one project need a version of package that is completely different from the other one, virtualenv helps us to resolve such kind of issues. It also helps to prevent polluting global site package.

virtualenv is just a package available at pypi, you can use pip to install virtualenv.

Question 258

How to install virtualenv?

Answer 258

pip3 install virtualenv

After installation you may need to add C:\Python34\Scripts to your PATH environment variable. This way commands like pip, virtualenv will become available in any directory level.

Question 259

How to create a virtualenv in a folder with your project?

Answer 259

mkdir new_project
cd new_project

virtualenv my_env

(This will create a new folder my_env inside python_project. This folder will contain a copy of python executables and pip library used to install packages. Here we have used my_env as name, but you can use anything you want. Now your virtual environment is ready to use, you just need to activate it.)

source my_env/bin/activate

After issuing the above command your command prompt string will change and will look something like,

( my_env ) Path_to_the_project: $

ANOTHER WAY (my PC):

mkdir new_project
python3.10 -m venv .venv/app
source .venv/app/bin/activate

Question 260

How to deactivate virtualenv?

Answer 260

To deactivate virtual environment you need to use the following command.

deactivate

Question 261

What is recursion?

Answer 261

When a function call itself is knows as recursion. Recursion works like loop but sometimes it makes more sense to use recursion than loop. You can convert any loop to recursion.

Question 262

What is base condition in recursive function?

Answer 262

It is a condition that stops recursion

Here is how recursion works. A recursive function calls itself. As you you’d imagine such a process would repeat indefinitely if not stopped by some condition. This condition is known as base condition. A

Question 263

What this error message means:
‘RuntimeError: maximum recursion depth exceeded in comparison’

Answer 263

This happens because python stop calling recursive function after 1000 calls by default. To change this behavior you need to amend the code as follows.

import sys
sys.setrecursionlimit(3000)

def fact(n):
if n == 0:
return 1
else:
return n * fact(n-1)

print(fact(2000))

Question 264

Every module in Python has a special attribute called ____

Answer 264

__name__

Question 265

The value of __name__ attribute is set to _____ when module run as main program

Answer 265

‘__main__’

Question 266

Python allows you to create anonymous function i.e function having no names using a facility called ________

Answer 266

lambda function.

Question 267

Lambda function

Answer 267

Lambda functions are small functions usually not more than a line.
- lambda function has no name
- the body of lambda function consists of only one expression
e.g result = lambda x, y: x*y
expression after colon is the body of the lambda function
- there is no need for any return statement in lambda function.

Question 268

Convert following expression into a lambda function:

def multiply(x, y):
return x * y

Answer 268

result = lambda x, y: x*y

result(6*8)

You don’t need to assign lambda function to a variable, you can use parentheses instead:

(lambda x, y: x * y)(6, 8)

Question 269

format()

Answer 269

The format() function formats a specified value into a specified format.

Syntax:
format(value, format)

Question 270

format codes

Answer 270

If you want to specify precision in floating point number use following syntax:

{[argument_index_or_keyword]:[width][.precision][type]}

np.

“Floating point pi = {0:.3f}, with {1:d} digit precision”.format(math.pi, 3)

0,1 to indeksy argumentów funkcji format()
.2f oznacza, że chcemy precyzje do dwóch miejsc po przecinku

“Floating point {0:10.3f}”.format(math.pi)

0 to indeks argumentu funkcji format() (jest tylko 1)
10 width
.3f precyzja do trzech miejsc po przecinku

You need to specify precision only in case of floating point numbers if you specify precision for integer ValueError will be raised.

In old code you can find % instead of {}

np.
“%d pens cost = %.2f” % (12, 150.87612)

> 12 pens cost = 150.88

Question 271

abs()

Answer 271

abs() function returns the absolute value (just magnitude without sign) of the number

e.g
»> abs(-45)
45

Question 272

bin()

Answer 272

bin() function returns the binary representation of the integer as a string

e.g
bin(4) # convert decimal to binary
> ‘0b100’

bin(0xff) # convert hexadecimal to binary, 0xff is same decimal 255
‘> 0b11111111’

Question 273

id()

Answer 273

id() function returns a memory address of the object

We can use this identifier to determine whether two objects are same or not.

e.g
a = 10
b = 5

> (10919712, 10919552)

id(a), id(b)

Question 274

map()

Answer 274

convert the map into a list, for readability:

map() function executes a specified function for each item in an iterable. The item is sent to the function as a parameter.

Syntax: map(function, iterables)

function - Required. The function to execute for each item

iterable - Required. A sequence, collection or an iterator object. You can send as many iterables as you like, just make sure the function has one parameter for each iterable.

e.g
def myfunc(a, b):
return a + b

x = map(myfunc, (‘apple’, ‘banana’, ‘cherry’), (‘orange’, ‘lemon’, ‘pineapple’))

print(x)

print(list(x))

> def myfunc(a, b):
return a + b

x = map(myfunc, (‘apple’, ‘banana’, ‘cherry’), (‘orange’, ‘lemon’, ‘pineapple’))

print(x)

print(list(x))

def myfunc(a, b):
return a + b
​
x = map(myfunc, (‘apple’, ‘banana’, ‘cherry’), (‘orange’, ‘lemon’, ‘pineapple’))
​
print(x)
​
#convert the map into a list, for readability:
print(list(x))
​
<map object at 0x034244F0>
[‘appleorange’, ‘bananalemon’, ‘cherrypineapple’]

Question 275

zip()

Answer 275

use the tuple() function to display a readable version of the result:

zip() function returns a zip object, which is an iterator of tuples where the first item in each passed iterator is paired together, and then the second item in each passed iterator are paired together etc.

a = (“John”, “Charles”, “Mike”)
b = (“Jenny”, “Christy”, “Monica”)

x = zip(a, b)

print(tuple(x))
> ((‘John’, ‘Jenny’), (‘Charles’, ‘Christy’), (‘Mike’, ‘Monica’))

Question 276

filter()

Answer 276

Funkcja filter() tworzy nową listę elementów na podstawie wejściowej listy elementów, wybierając tylko te wartości, dla których funkcja testując zwróci prawdę (True)

Question 276

reduce()

Answer 276

reduce() function accepts a function and a sequence and returns a single value calculated as follows:

1. Initially, the function is called with the first two items from the sequence and the result is returned.
2. The function is then called again with the result obtained in step 1 and the next value in the sequence. This process keeps repeating until there are items in the sequence.

e.g
from functools import reduce

def do_sum(x1, x2):
return x1 + x2

print(reduce(do_sum, [1, 2, 3, 4]))

> 10

Question 277

sorted()

Answer 277

built-in function allows us to sort the data. It accepts an iterable and returns a sorted list containing the items from the iterable. By default, it sorts in ascending order.

The syntax of sorted() function is as follows:

Syntax: sorted(iterable, key=None, reverse=False)

iterable - (required) Iterable to sort like string, list, dictionary, tuple etc.
key (optional) - It refers to the single argument function to customize the sort order. The function is applied to each item on the iterable. By default, this argument is set to None.
reverse - (optional) A boolean flag to reverse the sorting order. It defaults to False.

e.g
fruits = [‘lime’, ‘blueberry’, ‘plum’, ‘avocado’]

> [‘avocado’, ‘blueberry’, ‘lime’, ‘plum’]

sorted(fruits, key=len) # sort by string length
[‘lime’, ‘plum’, ‘avocado’, ‘blueberry’]

> [‘lime’, ‘plum’, ‘avocado’, ‘blueberry’]

Note that the sorted() returns a new list containing items from the iterable. It doesn’t change the original iterable in the process.

Notice that in the result of the first sorted() call A come before a. This is because the ASCII value of A is 65 and that of a is 97. For the same reason space character (‘ ‘), ASCII value of 32 comes before A.

Question 278

enumerate()

Answer 278

enumerate() function takes an iterable and returns an enumerate object (an iterator) that produces a tuple of the form (index, value), where index refers to the offset of the item and item refers to the corresponding item from the iterable.

Syntax:
enumerate(iterable[, start=0]

iterable - (required) Any iterable object like string, list, dictionary, etc.

start -initial value of the index. It defaults to 0.

e,g
list(enumerate(“hello”))
> [(0, ‘h’), (1, ‘e’), (2, ‘l’), (3, ‘l’), (4, ‘o’)]

for index, value in enumerate(“hello”):
print(index, value)

> 0 h
1 e
2 l
3 l
4 o

Question 278

reversed()

Answer 278

reversed() function allows us to process the items in a sequence in reverse order. It accepts a sequence and returns an iterator.

Syntax:
reversed(sequence)

sequence -A sequence list string, list, tuple etc.

print( reversed([44, 11, -90, 55, 3]) )
> <list_reverseiterator object at 0x7f9567dcdeb8>

print(list(reversed([44, 11, -90, 55, 3]))) # reversing a list
> [3, 55, -90, 11, 44]

To reverse user-defined objects the class must do one of the following:
1. Implement __len__() and __getitem__() methods

2. Implement __reversed__() method

Question 279

range()

Answer 279

range() function is used to generate a sequence of numbers over time. At its simplest, it accepts an integer and returns a range object (a type of iterable).

Syntax:
range(start, stop, step)

start (optional) Starting point of the sequence. It defaults to 0.
stop (required) Endpoint of the sequence. This item will not be included in the sequence.
step (optional) Step size of the sequence. It defaults to 1.

Here range() is called with two arguments, 5 and 10. As a result, it will generate a sequence of numbers from 5 up to 10 (but not including 10).

e.g

print(list(range(5, 10)))
> [5, 6, 7, 8, 9]

print(list(range(1, 20, 3)))
> [1, 4, 7, 10, 13, 16, 19]

for i in range(5):
print(i)

> 0
1
2
3
4

Question 280

sum()

Answer 280

function takes an iterable and returns the sum of items in it.

e.g.

sum([1, 2, 3, 4, 5]) # sum values in a list
> 15

It does sum elements of a list, tuple, set and dict. In dict the sum() adds the keys in the dictionary, ignoring its values.

Question 281

max()

Answer 281

max() function returns the largest of the input values. It accepts iterable (list, tuple, dict). It can accept key argument.

Syntax:
max(iterable, default=obj, key=func)

iterable (required) An iterable object like string, list, tuple etc.

default (optional) The default value to return if the iterable is empty.

key (optional) It refers to the single argument function to customize the sort order. The function is applied to each item on the iterable.

e.g
print(max((“python”, “lua”, “ruby”), key=len))

> python

Question 282

min()

Answer 282

function returns the smallest of the input values.

Syntax:
min(iterable[, default=obj, key=func]) -> value

iterable (required) An iterable object like string, list, tuple etc.
default (optional) - The default value to return if the iterable is empty.
key (optional) - It refers to the single argument function to customize the sort order. The function is applied to each item on the iterable.

min([2, -1, 4, 3])
> -1

min((“java”, “python”, “z++”), key=len)
‘z++’
> ‘z++’

Question 283

eval()

Answer 283

eval() allows us to execute arbitrary strings as Python code. It accepts a source string and returns an object

Syntax:
eval(expr, globals=None, locals=None)

expr (required) can be any valid Python expression
globals (optional) Global namespace to use while executing the source. It must be a dictionary. If not provided then the current global namespace will be used.
locals (optional) Local namespace to use while executing the source. It can be any mapping. If omitted, it defaults to globals dictionary.

eval(“5 == 5”)
> True

eval(“4 < 10”)
> True

eval(“‘hello’ + ‘py’”)
> ‘hellopy’

The eval() is not just limited to simple expression. We can execute functions, call methods, reference variables and so on.

eval(“abs(-11)”)
> 11

globals = {
‘a’: 10,
‘fruits’: [‘mangoes’, ‘peaches’, ‘ bananas’],
}

locals = {}

eval(“str(a) + ‘ ‘ + fruits[0]”, globals, locals)

> ‘10 mangoes’

The key takeaway is that only use eval() with the trusted source.

Question 284

len()

Answer 284

len() function counts the number of items in an object.

Syntax is as follows:
len(obj) -> length

obj can be a string, list, dictionary, tuple etc.

e.g

len({“spande”, “club”, “diamond”, “heart”}) # length of set
> 4

def gen_func():
for i in range(5):
yield i
len(gen_func())
> Traceback (most recent call last):
File “main.py”, line 5, in <module>
len(gen_func())
TypeError: object of type 'generator' has no len()</module>

As you can see above the len() function doesn’t work with a generator. Trying to call len() on a generator object will result in TypeError exception

To use len() on user-defined objects you will have to implement the __len__() method.

e.g
class Stack:

def \_\_init\_\_(self):
     self._stack = []
 
def push(self, item):
    self._stack.append(item)

 def pop(self):
     self._stack.pop()

 def \_\_len\_\_(self):
    return len(self._stack)

Question 285

ord()

Answer 285

ord() function (short of ordinal) returns an integer representing the character passed to it.

For ASCII characters, the returned value is 7-bit ASCII code, and for Unicode characters, it refers to the Unicode code point.

e.g
ord(“A”)
> 65

ord(“😀”) # Grinning Face
> 128512

To convert an integer returned by ord() back to its character equivalent we use chr() function.

Question 286

chr()

Answer 286

chr() function returns a single character string represented by the integer ordinal value

chr(65)
> ‘A

chr(128512) # Grinning Face
‘😀’

Question 287

any()

Answer 287

any() function tests whether any item in the iterable evaluates to True to not. It accepts an iterable and returns True, if at least one item in the iterable is true, otherwise, it returns False.

Syntax:
any(iterable) -> boolean

e.g
any([10, “”, “one”])
> True

any((“”, {}))
> False

any([])
> False

Question 288

globals()

Answer 288

globals() function returns a dictionary containing the variables defined in the global namespace. When globals() is called from a function or method, it returns the dictionary representing the global namespace of the module where the function or method is defined, not from where it is called

zwraca słownik zawierający zmienne zdefiniowane globalnie

e.g

from pprint import pprint

a = 100
b = 4

def foo():
x = 100 # x is a local variable

pprint(globals())

> ‘a’: 100,
‘b’: 4
…

Question 289

locals()

Answer 289

locals() function returns a dictionary containing the variables defined in the local namespace. Calling locals() in the global namespace is same as calling globals() and returns a dictionary representing the global namespace of the module.

zwraca słownik zawierający zmienne zdefiniowane lokalnie

Question 290

all()

Answer 290

all() function tests whether all items in the iterable evaluates to True or not. It accepts an iterable and returns True if all the item is true, otherwise, it returns False.

e.g

all([‘alpha’, ‘beta’, ‘’])
> False

all([‘one’, ‘two’, ‘three’])
> True

Question 291

MySQLdb

Answer 291

MySQLdb is an api for accessing MySQL database using python. It is built on top of MySQL C API.

MySQLdb don’t yet have support for python 3, it supports only python 2.4 - 2.7 . As a result you need to use python 2 for this tutorial. We will be using python 2.7.9, which you can download from here.

Question 292

There are two types of files you can open and work with in Python:

Answer 292

• Text files
• Binary files

A text file is simply a file which stores sequences of characters using an encoding like utf-8, latin1 etc., whereas in the case of binary file data is stored in the same format as in Computer memory.

Text files: Python source code, HTML file, text file, markdown file etc.

Binary files: executable files, images, audio etc.

It is important to note that inside the disk both types of files are stored as a sequence of 1s and 0s. The only difference is that when a text file is opened the data is decoded back using the same encoding scheme they were encoded in. However, in the case of binary files no such thing happens.

Question 293

open()

Answer 293

built-in function is used to open the file. Its syntax is as follows:
open(filename, mode) -> file object

filename (requires) absolute or relative path of the file to be opened.

mode (optional) mode is a string which refers to the processing mode (i.e read, write, append etc;) and file type.

Modes:
r Open the file for reading (default).
w Open the file for writing.
a Open the file in append mode i.e add new data to the end of the file.
r+ Open the file for reading and writing both
x Open the file for writing, only if it doesn’t already exist.

We can also append t or b to the mode string to indicate the type of the file we will be working with. The t is used for text file and b for binary files. If neither specified, t is assumed by default:

e.g
open(‘todo.md’, ‘rt’)

Question 294

read([n])

Answer 294

= open(“poem.txt”, “r”)
f.read(3) # read the first 3 characters
>’The’

Reads and returns n bytes or less (if there aren’t enough characters to read) from the file as a string. If n not specified, it reads the entire file as a string and returns it.

Question 295

readline()

Answer 295

Reads and returns the characters until the end of the line is reached as a string.

= open(“poem.txt”, “r”)
f.read(4) # read the first 4 characters
>’The ‘

f.readline() # read until the end of the line is reached
> ‘caged bird sings\n’

Question 296

readlines()

Answer 296

= open(“poem.txt”, “r”)

f.readlines()
> [‘The caged bird sings\n’, ‘with a fearful trill\n’, ‘of things unknown\n’, ‘but longed for still\n’]

Question 297

How to read file in chunks?

Answer 297

Reading File in Chunks :

f = open(“poem.txt”, “r”)

chunk = 200

while True:
data = f.read(chunk)
if not data:
break
print(data)

> The caged bird sings
with a fearful trill
of things unknown
but longed for still

Question 298

write(s)

Answer 298

Writes the string s to the file and returns the number characters written.
e.g
f.write(“When I think about myself, “)
> 26

Unlike the print() function the write() method doesn’t add a newline character (\n) at the end of the line. If you want the newline character you have to add it manually.

Question 299

writelines(s)

Answer 299

Writes all strings in the sequence s to the file.
e.g
»> lines = [
“Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod”,
“tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam,”
“quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo”,
“consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse”,
“cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non”,
“proident, sunt in culpa qui officia deserunt mollit anim id est laborum.”
]

f = open(“lorem.txt”, “w”)
f.writelines(lines)
f.close()

Notice that unlike the print() function the write() method doesn’t add a newline character (\n) at the end of the line. If you want the newline character you have to add it manually (adding \n at the end of each line)

Question 300

What is buffering?

Answer 300

The open() method provides an optional third argument to control the buffer.

Buffering is the process of storing data temporarily in the PC memory before it is moved to a new location (e.g. a disk)

When we use a buffer, the data is written to the disk only when the buffer becomes full or when the close() method is called.

This process is called flushing the output. You can also flush the output manually using the flush() method of the file object. Note that the flush() only saves the buffered data to the disk. It doesn’t close the file.

Question 301

Reading and writing binary file is done by appending ___ to the mode string.

Answer 301

b
In Python 3, the binary data is represented using a special type called bytes.

The bytes type represents an immutable sequence of numbers between 0 and 255.

e.g
binary_poem = bytes(open(“poem.txt”).read(), encoding=”utf-8”)

e.g
f = open(“binary_poem”, “wb”)
f.write(binary_poem)
> 80
f.close()

with binary data we can’t reliably use readline() and file object (as an iterator) to read the contents of a file because might be no newline character in a file. The best way to read binary data is to read it in chunks using the read() method

Question 302

How to read and write file at specific locations using Python?

Answer 302

To achieve this we have to use following methods:
tell() which returns the current position of the file pointer.
seek(offset, [whence=0]) which moves the file pointer to the given offset.

offset - określa gdzie jest ustawiony file pointer. Wyraża się go w liczbie bajtów
whence - skąd (domyślnie 0, czyli od początku pliku)

When we move the pointer the next read (or write) operation will start from this point.

Możemy użyć f.seek() to przesunięcia file pointera na początek pliku w następujący sposób:

f.seek(0) # rewind the file pointer to the beginning, same as seek(0, 0)
> 0

Potem możemy sprawdzić za pomocą f.tell() czy na pewno dobrze się przesunęliśmy,

f.tell()
> 0

Question 303

What is writing text/binary file in python?

Answer 303

To write to an existing file, you must add a parameter to the open() function:

“a” - Append - will append to the end of the file

“w” - Write - will overwrite any existing content

Question 304

following three calls to open() are equivalent or not?

Answer 304

open file todo.md for reading in text mode

open(‘todo.md’)
open(‘todo.md’, ‘r’)
open(‘todo.md’, ‘rt’)

Yes, because ‘r’ (read mode) and ‘t’ (text file) are default.

The mode is optional, if not specified then the file will be opened as a text file for reading only.Note that before you can read a file, it must already exist, otherwise open() will raise FileNotFoundError exception. However, if you open a file for writing (using mode such as w, a, or r+), Python will automatically create the file for you. If the file already exists then its content will be deleted. If you want to prevent that open the file in x mode.

Question 305

close()

Answer 305

Although, the file is automatically closed when the program ends but it is still a good practice to do so explicitly. Failing to close the file in a large program could be problematic and may even cause the program to crash.

To close the file call the close() method of the file object. Closing the file frees up resources associated with it and flushes the data in the buffer to the disk.

Question 306

What is a file pointer?

Answer 306

When you open a file via the open() method. The operating system associates a pointer that points to a character in the file. The file pointer determines from where the read and write operation will take place.

Question 307

EOF

Answer 307

End Of the File

Question 308

tell()

Answer 308

Returns the current position of the file pointer.

Question 309

seek(offset, [whence=0])

Answer 309

Moves the file pointer to the given offset. The offset refers to the byte count and whence determines the position relative to which the offset will move the file pointer.

The default value of whence is 0, which means that offset will move the file pointer from the beginning of the file. If whence is set to 1 or 2, the offset will move the file’s pointer from the current position or from the end of the file, respectively.

Question 310

we can use _____ method to rewind the file pointer to the beginning of the file

Answer 310

fseek()

e.g

f.seek(0) # rewind the file pointer to the beginning, same as seek(0, 0)
> 0

f.tell()
> 0

f.read(14) # read the first 14 characters
> b’The caged bird’

f.tell()
> 14

Question 311

To move the file pointer from 12 bytes forward from current position call ______ method

Answer 311

seek()

f.tell()
> 14

f.seek(12, 1)
> 26

f.tell()
> 26

We can also move the file pointer backward. For example, the following call to seek() moves the file pointer 13 bytes backward from the current position.

f.seek(-13, 1)
> 13

Question 312

The _____ allows us to automatically close the file once we are done working with it.

Answer 312

with statement

e.g
with open(‘poem.txt’) as f:
print(f.read()) # read the entire file

Question 313

What is CSV File?

Answer 313

CSV (Comma-separated values) is a common data exchange format used by the applications to produce and consume data. Some other well-known data exchange formats are XML, HTML, JSON etc.

A CSV file is a simple text file where each line contains a list of values (or fields) delimited by commas.

Although the term “Comma” appears in the format name itself, but you will encounter CSV files where data is delimited using tab (\t) or pipe (|) or any other character that can be used as a delimiter.

The first line of the CSV file represents the header containing a list of column names in the file. The header is optional but highly recommended.

Question 314

Using commas in CSV files as delimiter:

If the values in your CSV file contains commas, then it must be enclosed inside ___________

Answer 314

double quotes

Name,Age,Address
Jerry,10,”2776 McDowell Street, Nashville, Tennessee”
Tom,20,”3171 Jessie Street, Westerville, Ohio”
Mike,30,”1818 Sherman Street, Hope, Kansas”

Question 315

If you have double quotes embedded inside a field, it must be escaped with another __________ . Otherwise, they will not be interpreted correctly.

Answer 315

double quotes

e.g
Id,User,Comment
1,Bob,”John said ““Hello World”””
2,Tom,”“The Magician””

Question 316

The ____ module is used for reading and writing CSV files. It mainly provides following classes and functions:

reader()
writer()
DictReader()
DictWriter()

Answer 316

csv

Question 317

csv module
reader()

Answer 317

The reader() function takes a file object and returns a _csv.reader object that can be used to iterate over the contents of a CSV file.

reader(fileobj, dialect=’excel’, fmtparam)

fileobj - (required) It refers to the file object, which we want to read
dialect - (optional) Dialect refers to the different ways of formatting the CSV document. By default, the csv module uses the same format as Microsoft Excel.
fmtparam - (optional) It refers to the set of keyword arguments to customize the dialect (see the next section).

e.g
import csv

with open(‘employees.csv’, ‘rt’) as f:
csv_reader = csv.reader(f)

for line in csv_reader:
    print(line)

> [‘id’, ‘name’, ‘email’, ‘age’, ‘designation’]
[‘1’, ‘John’, ‘john@mail.com’, ‘24’, ‘programmer’]
[‘2’, ‘Bob’, ‘bob@mail.com’, ‘34’, ‘designer’]
[‘3’, ‘Mary’, ‘mary@mail.com’, ‘43’, ‘sales’]

Notice that each line in the CSV file is returned as a list of strings.

Question 318

Hoe to get data from certain fields using csv.reader() method?

Answer 318

Because each line in the CSV file is returned as a list of strings, to get the data from certain fields, you can use indexing.

e.g
import csv

with open(‘employees.csv’, ‘rt’) as f:
csv_reader = csv.reader(f)

for line in csv_reader:
    print(line[0], line[1], line[2])

> id name email
1 John john@mail.com
2 Bob bob@mail.com
3 Mary mary@mail.com

Question 319

How to skip the heading when reading a csv file?

Answer 319

If you want to skip heading call the next() built-in function on the _csv.reader object and then loop over the remaining lines as usual.

e.g
import csv

with open(‘employees.csv’, ‘rt’) as f:
csv_reader = csv.reader(f)

next(csv_reader) # skip the heading

for line in csv_reader:
    print(line[0], line[1], line[2])

> 1 John john@mail.com
2 Bob bob@mail.com
3 Mary mary@mail.com

Question 320

How to modify the dialect of csv files when using csv module?

Answer 320

By default, the csv module works according to the dialect used by Microsoft excel, but you can also define your own format using some additional arguments that you can pass to the reader() function to customize its working

e.g
delimiter - It refers to the character used to separate values (or fields) in the CSV file. It defaults to comma (,).

skipinitialspace - It controls how the space following the delimiter will be interpreted. If True , the initial whitespaces will be removed. It defaults to False

lineterminator - It refers to the character sequence used to terminate the line. It defaults to \r\n.

quotechar - It refers to the single character string that will be used to quote values if special characters (like delimiter) appears inside the field. It defaults to “.

quoting - controls when quotes should be generated by the writer or recognized by the reader. It can take one of the following constants:
csv.QUOTE_MINIMAL means add quote only when required, for example, when a field contains either the quotechar or the delimiter. This is the default.
csv.QUOTE_ALL means quotes everything regardless of the field type.
csv.QUOTE_NONNUMERIC means quotes everything except integers and floats.
csv.QUOTE_NONE means that do not quote anything on output. However, while reading quotes are included around the field values.

Question 321

How to change delimiter using csv.reader() function?

Answer 321

you pass delimiter argument to the reader() function

e.g
import csv

with open(‘employees.csv’, ‘rt’) as f:
csv_reader = csv.reader(f, delimiter=’|’)

for line in csv_reader:
    print(line)

> [‘id’, ‘name’, ‘email’, ‘age’, ‘designation’]
[‘1’, ‘John’, ‘john@mail.com’, ‘24’, ‘programmer’]
[‘2’, ‘Bob’, ‘bob@mail.com’, ‘34’, ‘designer’]
[‘3’, ‘Mary’, ‘mary@mail.com’, ‘43’, ‘sales’]

Question 322

How to remove spaces following the delimiter when reading csv file?

Answer 322

you pass skipinitialspace argument to the reader() function

e.g
import csv

with open(‘baseball_players.csv’, ‘rt’) as f:
csv_reader = csv.reader(f, skipinitialspace=True)

Question 323

When reading a csv file, how to change quote character to a single quote ?

Answer 323

you pass quotechar argument to the reader() function

e.g
import csv

with open(‘housing.csv’, ‘rt’) as f:
csv_reader = csv.reader(f, skipinitialspace=True, quotechar=”’”)

for line in csv_reader:
    print(line)

Question 324

Sometimes, a csv files use some character (e.g /) as something that indicates the reader that it should escape embedded double quotes.
e.g we face something like this:

> [‘Id’, ‘User’, ‘Comment’]
[‘1’, ‘Bob’, ‘John said \Hello World\””’]
[‘2’, ‘Tom’, ‘\The Magician\””’]
[‘3’, ‘Harry’, ‘\walk around the corner\” she explained to the child”’]
[‘4’, ‘Louis’, ‘He said, \stop pulling the dog's tail\””’]

This output is certainly not desirable. To get the correct output change the escape character using _____ argument of _____ method

Answer 324

escapechar argument, csv.reader() method

e.g.
import csv

with open(‘employees.csv’, ‘rt’) as f:
csv_reader = csv.reader(f, skipinitialspace=True, escapechar=’\’)

for line in csv_reader:
    print(line)

> [‘Id’, ‘User’, ‘Comment’]
[‘1’, ‘Bob’, ‘John said “Hello World”’]
[‘2’, ‘Tom’, ‘“The Magician”’]
[‘3’, ‘Harry’, ‘“walk around the corner” she explained to the child’]
[‘4’, ‘Louis’, ‘He said, “stop pulling the dog's tail”’]

Question 325

Some files use double quote to escape the embedded double quote characters in the field.

e.g
Id, Actor, Dialogue
1, Harley Betts, “The suspect told the arresting officer, ““I was nowhere near the crime.”””
2, Clyde Esparza, “John said, ““I have just finished reading Browning’s ‘My Last Duchess.’”””
3, Zack Campbell, “Bill asked Sandra, ““Will you marry me?”””
4, Keziah Chaney, “The librarian whispered to us, ““The sign on the wall says ‘Quiet’”””

To change it set _______ argument of _______ method to _______

Answer 325

doublequote argument, csv.reader() method, True

e.g
import csv

with open(‘employees.csv’, ‘rt’) as f:
# same as csv_reader = csv.reader(f, skipinitialspace=True)
csv_reader = csv.reader(f, skipinitialspace=True, doublequote=True)

for line in csv_reader:
print(line)

> [‘Id’, ‘Actor’, ‘Dialogue’]
[‘1’, ‘Harley Betts’, ‘The suspect told the arresting officer, “I was nowhere near the crime.”’]
[‘2’, ‘Clyde Esparza’, ‘John said, “I have just finished reading Browning's 'My Last Duchess.'”’]
[‘3’, ‘Zack Campbell’, ‘Bill asked Sandra, “Will you marry me?”’]
[‘4’, ‘Keziah Chaney’, ‘The librarian whispered to us, “The sign on the wall says 'Quiet'”’]

Question 326

To write data to a CSV file we use the _____ function from ____ module

Answer 326

writer() function, csv module

It accepts the same argument as the reader()
fileobj - (required) It refers to the file object
dialect - (optional) Dialect refers to the different ways of formatting the CSV document. By default, the csv module uses the same format as Microsoft Excel.
fmtparam - (optional) Formatting parameters, work same as the reader()’s function.

The writer instance provides the following two methods to write data:
writerow(row) - Writes a single row of data and returns the number of characters written. The row must be a sequence of strings and number.

writerows(rows) - Writes multiple rows of data and returns None. The rows must be a sequence.

Question 327

How to use writerow() and writerows methods for a csv file?

Answer 327

import csv

header = [‘id’, ‘name’, ‘address’, ‘zip’]
rows = [
[1, ‘Hannah’, ‘4891 Blackwell Street, Anchorage, Alaska’, 99503 ],
[2, ‘Walton’, ‘4223 Half and Half Drive, Lemoore, California’, 97401 ],
[3, ‘Sam’, ‘3952 Little Street, Akron, Ohio’, 93704],
[4, ‘Chris’, ‘3192 Flinderation Road, Arlington Heights, Illinois’, 62677],
[5, ‘Doug’, ‘3236 Walkers Ridge Way, Burr Ridge’, 61257],
]

with open(‘customers.csv’, ‘wt’) as f:
csv_writer = csv.writer(f)

csv_writer.writerow(header) # write header

csv_writer.writerows(rows)

Question 328

csv module
How to write a multiple rows using writerows()?

Answer 328

import csv

header = [‘id’, ‘name’, ‘address’, ‘zip’]
rows = [
[1, ‘Hannah’, ‘4891 Blackwell Street, Anchorage, Alaska’, 99503 ],
[2, ‘Walton’, ‘4223 Half and Half Drive, Lemoore, California’, 97401 ],
[3, ‘Sam’, ‘3952 Little Street, Akron, Ohio’, 93704],
[4, ‘Chris’, ‘3192 Flinderation Road, Arlington Heights, Illinois’, 62677],
[5, ‘Doug’, ‘3236 Walkers Ridge Way, Burr Ridge’, 61257],
]

with open(‘customers.csv’, ‘wt’) as f:
csv_writer = csv.writer(f)

csv_writer.writerow(header) # write header

for row in rows:
    csv_writer.writerow(row)

Question 329

In the following example:

customers.csv

> id,name,address,zip
1,Hannah,”4891 Blackwell Street, Anchorage, Alaska”,99503
2,Walton,”4223 Half and Half Drive, Lemoore, California”,97401
3,Sam,”3952 Little Street, Akron, Ohio”,93704
4,Chris,”3192 Flinderation Road, Arlington Heights, Illinois”,62677
5,Doug,”3236 Walkers Ridge Way, Burr Ridge”,61257

only the address field is wrapped around double quotes. What can be the cause?

Answer 329

This is because by default the quoting argument of the csv.writer() method is set to QUOTE_MINIMAL.

In other words, fields will be quoted only when quotechar or delimiter appears in the data.

If you want double quotes around all textual data set quoting argument to QUOTE_NONNUMERIC.

e.g
with open(‘customers.csv’, ‘wt’) as f:
csv_writer = csv.writer(f, quoting=csv.QUOTE_NONNUMERIC)

> “id”,”name”,”address”,”zip”
1,”Hannah”,”4891 Blackwell Street, Anchorage, Alaska”,99503
2,”Walton”,”4223 Half and Half Drive, Lemoore, California”,97401
3,”Sam”,”3952 Little Street, Akron, Ohio”,93704
4,”Chris”,”3192 Flinderation Road, Arlington Heights, Illinois”,62677
5,”Doug”,”3236 Walkers Ridge Way, Burr Ridge”,61257

Question 330

If you want to quote all fields regardless of data type set ______ to _______

Answer 330

set quoting to csv.QUOTE_ALL

e. g.
import csv

with open(‘customers.csv’, ‘wt’) as f:
csv_writer = csv.writer(f, quoting=csv.QUOTE_ALL)

> “id”,”name”,”address”,”zip”
“1”,”Hannah”,”4891 Blackwell Street, Anchorage, Alaska”,”99503”
“2”,”Walton”,”4223 Half and Half Drive, Lemoore, California”,”97401”
“3”,”Sam”,”3952 Little Street, Akron, Ohio”,”93704”
“4”,”Chris”,”3192 Flinderation Road, Arlington Heights, Illinois”,”62677”
“5”,”Doug”,”3236 Walkers Ridge Way, Burr Ridge”,”61257”

Question 331

How to change the quote character from double quote (“) to a single quote (‘) when using csv.writer()?

Answer 331

import csv

with open(‘customers.csv’, ‘wt’) as f:
csv_writer = csv.writer(f, quotechar=”’”)

> id,name,address,zip
1,Hannah,’4891 Blackwell Street, Anchorage, Alaska’,99503
2,Walton,’4223 Half and Half Drive, Lemoore, California’,97401
3,Sam,’3952 Little Street, Akron, Ohio’,93704
4,Chris,’3192 Flinderation Road, Arlington Heights, Illinois’,62677
5,Doug,’3236 Walkers Ridge Way, Burr Ridge’,61257

Question 332

How to turn off quoting all-together when using csv.writer()?

Answer 332

We can turn off quoting all-together by setting quoting to csv.QUOTE_NONE. However, if you do that and delimiter character appears in the data then you will get an error like this:
> Traceback (most recent call last):
…
csv_writer.writerows(rows)
_csv.Error: need to escape, but no escapechar set

The problem is that the address field contains embedded commas (,) and since we have turned off the ability to quote fields, the csv module doesn’t know how to escape them properly.

Question 333

csv module
How to escape fields properly, when we just used csv.QUOTE_NONE?

Answer 333

This where the escapechar argument comes into play. We have to set the escapechar to e. g backslash ():

import csv

with open(‘customers.csv’, ‘wt’) as f:
csv_writer = csv.writer(f, quoting=csv.QUOTE_NONE, escapechar=’\’)

> id,name,address,zip
1,Hannah,4891 Blackwell Street\, Anchorage\, Alaska,99503
2,Walton,4223 Half and Half Drive\, Lemoore\, California,97401
3,Sam,3952 Little Street\, Akron\, Ohio,93704
4,Chris,3192 Flinderation Road\, Arlington Heights\, Illinois,62677
5,Doug,3236 Walkers Ridge Way\, Burr Ridge,61257

Question 334

What is DictReader?

Answer 334

DictReader is used for reading a csv files. It works almost exactly like reader() but instead of retuning a line as a list, it returns a dictionary.

Syntax:
DictReader(fileobj, fieldnames=None, restkey=None, restval=None, dialect=’excel’, fmtparam)

fileobj - (required) It refers to the file object.
fieldnames - (optional) It refers to the list of keys that will be used in the returned dictionary in order. If omitted, the field names are inferred from the first row of the CSV file.
restkey - (optional) If the row has more fields than specified in the fieldnames parameter, then the remaining fields is stored as a sequence keyed by the value of restkey argument.
restval - (optional) It provides value to fields which are missing from the input.
dialect - (optional) Dialect refers to the different ways of formatting the CSV document. By default, the csv module uses the same format as Microsoft excel.
fmtparam - It refers to formatting arguments and works exactly like reader() and writer()

Question 335

How to use DictReader?

Answer 335

import csv

with open(‘customers.csv’, ‘rt’) as f:
csv_reader = csv.DictReader(f)

for row in csv_reader:
    print(row)

> ‘id’: ‘1’, ‘name’: ‘Hannah’, ‘zip’: ‘99503’, ‘address’: ‘4891 Blackwell Street, Anchorage, Alaska’}
{‘id’: ‘2’, ‘name’: ‘Walton’, ‘zip’: ‘97401’, ‘address’: ‘4223 Half and Half Drive, Lemoore, California’}
{‘id’: ‘3’, ‘name’: ‘Sam’, ‘zip’: ‘93704’, ‘address’: ‘3952 Little Street, Akron, Ohio’}
{‘id’: ‘4’, ‘name’: ‘Chris’, ‘zip’: ‘62677’, ‘address’: ‘3192 Flinderation Road, Arlington Heights, Illinois’}
{‘id’: ‘5’, ‘name’: ‘Doug’, ‘zip’: ‘61257’, ‘address’: ‘3236 Walkers Ridge Way, Burr Ridge’}

Headears become keys

Question 336

How to use fieldnames parameter in DictReader()?

Answer 336

import csv

with open(‘customers.csv’, ‘rt’) as f:
fields = [‘id’, ‘name’, ‘address’, ‘zip’]
csv_reader = csv.DictReader(f, fieldnames=fields)

for row in csv_reader:
    print(row)

> {‘name’: ‘Hannah’, ‘zip’: ‘99503’, ‘id’: ‘1’, ‘address’: ‘4891 Blackwell Street, Anchorage, Alaska’}
{‘name’: ‘Walton’, ‘zip’: ‘97401’, ‘id’: ‘2’, ‘address’: ‘4223 Half and Half Drive, Lemoore, California’}
{‘name’: ‘Sam’, ‘zip’: ‘93704’, ‘id’: ‘3’, ‘address’: ‘3952 Little Street, Akron, Ohio’}
{‘name’: ‘Chris’, ‘zip’: ‘62677’, ‘id’: ‘4’, ‘address’: ‘3192 Flinderation Road, Arlington Heights, Illinois’}
{‘name’: ‘Doug’, ‘zip’: ‘61257’, ‘id’: ‘5’, ‘address’: ‘3236 Walkers Ridge Way, Burr Ridge’}

Question 337

How to use restkey parameter in DictReader()?

Answer 337

We can use optional restkey parameter to a DictReader() function when the row has more fields than specified in the fieldnames parameter

e.g
import csv

with open(‘customers.csv’, ‘rt’) as f:
fields = [‘id’,’name’,]
csv_reader = csv.DictReader(f, fieldnames=fields, restkey=’extra’)

for row in csv_reader:
    print(row)

> {‘id’: ‘1’, ‘name’: ‘Hannah’, ‘extra’: [‘4891 Blackwell Street, Anchorage, Alaska’, ‘99503’]}
{‘id’: ‘2’, ‘name’: ‘Walton’, ‘extra’: [‘4223 Half and Half Drive, Lemoore, California’, ‘97401’]}
{‘id’: ‘3’, ‘name’: ‘Sam’, ‘extra’: [‘3952 Little Street, Akron, Ohio’, ‘93704’]}
{‘id’: ‘4’, ‘name’: ‘Chris’, ‘extra’: [‘3192 Flinderation Road, Arlington Heights, Illinois’, ‘62677’]}
{‘id’: ‘5’, ‘name’: ‘Doug’, ‘extra’: [‘3236 Walkers Ridge Way, Burr Ridge’, ‘61257’]}

Question 338

How to use restval parameter in DictReader()?

Answer 338

We can use optional resval parameter to a DictReader() function when we want to save the place for additional data which we don’t yet have.

e.g we have a csv file with id, name, and address of customers, but we want an additional place for email and phone

import csv

with open(‘customers.csv’, ‘rt’) as f:
fields = [‘id’,’name’, ‘address’, ‘zip’, ‘phone’, ‘email’] # two extra fields
csv_reader = csv.DictReader(f, fieldnames=fields, restkey=’extra’, restval=’NA’)

for row in csv_reader:
    print(row)

> {‘id’: ‘1’, ‘name’: ‘Hannah’, ‘email’: ‘NA’, ‘phone’: ‘NA’, ‘address’: ‘4891 Blackwell Street, Anchorage, Alaska’, ‘zip’: ‘99503’}
{‘id’: ‘2’, ‘name’: ‘Walton’, ‘email’: ‘NA’, ‘phone’: ‘NA’, ‘address’: ‘4223 Half and Half Drive, Lemoore, California’, ‘zip’: ‘97401’}
{‘id’: ‘3’, ‘name’: ‘Sam’, ‘email’: ‘NA’, ‘phone’: ‘NA’, ‘address’: ‘3952 Little Street, Akron, Ohio’, ‘zip’: ‘93704’}
{‘id’: ‘4’, ‘name’: ‘Chris’, ‘email’: ‘NA’, ‘phone’: ‘NA’, ‘address’: ‘3192 Flinderation Road, Arlington Heights, Illinois’, ‘zip’: ‘62677’}
{‘id’: ‘5’, ‘name’: ‘Doug’, ‘email’: ‘NA’, ‘phone’: ‘NA’, ‘address’: ‘3236 Walkers Ridge Way, Burr Ridge’, ‘zip’: ‘61257’}

Question 339

What is DictWriter?

Answer 339

The DictWriter object writes a dictionary to a CSV file. Its syntax is as follows:

DictWriter(fileobj, fieldnames, restval=’’, extrasaction=’raise’, dialect=’excel’, fmtparam)

fileobj - It refers to the file object
fieldnames - It refers to the field names and the order in which they will be written the file.
restval - It provides the missing value for the keys which doesn’t exist in the dictionary.
extrasaction - It controls what action to take if the dictionary contains a key, that is not found in the fieldnames argument. By default, extrasaction is set to raise, which means an exception will be raised in such an event. If you want to ignore the extra values set extrasaction to ignore.

The DictWriter provides the following three methods to write data.
writeheader()
writerow(row)
writerows(rows)

Question 340

writeheader()

Answer 340

DictWriter metod
Writes the header (i.e fieldnames) to the CSV file and returns None.

Question 341

writerow(row)

Answer 341

DictWriter metod
Writes a single row of data and returns the number of characters written. The row must be a sequence of strings and number.

Question 342

writerows(rows)

Answer 342

DictWriter metod
Writes multiple rows of data and returns None. The rows must be a sequence.

Question 343

How to use DictWriter()?

Answer 343

import csv

header = [‘id’, ‘name’, ‘address’, ‘zip’, ‘email’] # an extra field email

rows = [
{‘id’: 1, ‘name’: ‘Hannah’, ‘address’: ‘4891 Blackwell Street, Anchorage, Alaska’, ‘zip’: 99503 },
{‘id’: 2, ‘name’: ‘Walton’, ‘address’: ‘4223 Half and Half Drive, Lemoore, California’, ‘zip’: 97401 },
{‘id’: 3, ‘name’: ‘Sam’, ‘address’: ‘3952 Little Street, Akron, Ohio’, ‘zip’: 93704 },
{‘id’: 4, ‘name’: ‘Chris’, ‘address’: ‘3192 Flinderation Road, Arlington Heights, Illinois’, ‘zip’: 62677},
{‘id’: 5, ‘name’: ‘Doug’, ‘address’: ‘3236 Walkers Ridge Way, Burr Ridge’, ‘zip’: 61257},
]

with open(‘dictcustomers.csv’, ‘wt’) as f:

csv_writer = csv.DictWriter(f, fieldnames=header, restval="NA")
csv_writer.writeheader() # write header
csv_writer.writerows(rows)

Question 344

What to do, when we want to use DictWrite() on a csv file, but in this file we have more headers than we want in our dictionary?

Answer 344

To prevent the exception from being raised we have set extrasaction to ignore.

e.g
import csv

header = [‘id’, ‘name’, ‘address’] # notice zip is missing

rows = [
{‘id’: 1, ‘name’: ‘Hannah’, ‘address’: ‘4891 Blackwell Street, Anchorage, Alaska’, ‘zip’: 99503 },
{‘id’: 2, ‘name’: ‘Walton’, ‘address’: ‘4223 Half and Half Drive, Lemoore, California’, ‘zip’: 97401 },
{‘id’: 3, ‘name’: ‘Sam’, ‘address’: ‘3952 Little Street, Akron, Ohio’, ‘zip’: 93704 },
{‘id’: 4, ‘name’: ‘Chris’, ‘address’: ‘3192 Flinderation Road, Arlington Heights, Illinois’, ‘zip’: 62677},
{‘id’: 5, ‘name’: ‘Doug’, ‘address’: ‘3236 Walkers Ridge Way, Burr Ridge’, ‘zip’: 61257},
]

with open(‘dictcustomers.csv’, ‘wt’) as f:
csv_writer = csv.DictWriter(
f,
fieldnames=header,
restval=”NA”,
extrasaction=’ignore’ # ignore extra values in the dictionary
)
csv_writer.writeheader() # write header
csv_writer.writerows(rows)

Question 345

What is a Dialect when considering a csv file?

Answer 345

A dialect object or (simply dialect) is a way to group various formatting parameters. Once you have created the dialect object, simply pass it to the reader or writer, rather than passing each formatting argument separately.

To create a new dialect, we use register_dialect() function. It accepts dialect name as a string and one or more formatting parameters as keyword arguments.

Question 346

Consider formatting your csf file (creating a Dialect). Do you know what following terms mean? And what are their default values?

delimiter, skipinitialspace, lineterminator, quotechar, quoting, escapechar, doublequote

Answer 346

delimiter (,) - It refers to the character used to separate values (or fields) in the CSV file.

skipinitialspace (False) - It controls how the space following the delimiter will be interpreted. If True, the initial whitespaces will be removed.

lineterminator (\r\n) - It refers to the character sequence used to terminate the line.

quotechar (“) - It refers to the single character string that will be used to quote values if special characters (like delimiter) appears inside the field.

quoting (csv.QUOTE_NONE) - controls when quotes should be generated by the writer or recognized by the reader (see above for other options).

escapechar (None) - It refers to the one-character string used to escape the delimiter when quoting is set to csv.QUOTE_NONE

doublequote (True) - controls the handling of quotes inside fields. When True, two consecutive quotes are interpreted as one during read, and when writing, each quote character embedded in the data is written as two quotes.

Question 347

How to use register_dialect() function?

Answer 347

import csv

create and register new dialect
csv.register_dialect(‘psv’, delimiter=’|’, quoting=csv.QUOTE_NONNUMERIC)

header = [‘id’, ‘year’, ‘age’, ‘name’, ‘movie’]

rows = [
{‘id’: 1, ‘year’: 2013, ‘age’: 55, ‘name’: “Daniel Day-Lewis”, ‘movie’: “Lincoln” },
{‘id’: 2, ‘year’: 2014, ‘age’: 44, ‘name’: “Matthew McConaughey”, ‘movie’: “Dallas Buyers Club” },
{‘id’: 3, ‘year’: 2015, ‘age’: 33, ‘name’: “Eddie Redmayne”, ‘movie’: “The Theory of Everything” },
{‘id’: 4, ‘year’: 2016, ‘age’: 41, ‘name’: “Leonardo DiCaprio”, ‘movie’: “The Revenant” }
]

with open(‘oscars.csv’, ‘wt’) as f:

csv_writer = csv.DictWriter(
             f,
             fieldnames=header,
             dialect='psv', # pass the new dialect
             extrasaction='ignore'
)

csv_writer.writeheader() # write header

csv_writer.writerows(rows)

Question 348

What is JSON?

Answer 348

JSON (JavaScript Object Notation) is language-neutral data interchange format. It was created and popularized by Douglas Crockford. In its short history, JSON has become a defacto standard for data transfer across the web.

JSON is a text-based format which is derived from JavaScript object syntax. However, it is completely independent of JavaScript, so you don’t need to know any JavaScript to use JSON.

JSON is commonly used by web applications to transfer data between client and server. If you are using a web service then there are good chances that data will be returned to you in JSON format, by default.

JSON format closely resembles to dictionaries in Python.

Question 349

What is XML?

Answer 349

Before the inception of JSON, XML was predominantly used to send and receive data between the client and the server. The problem with XML is that that it is verbose, heavy and not easy to parse. However, this is not the case with JSON, as you will see soon.

Question 350

What is Serialization?

Answer 350

Serialization: The process of converting an object into a special format which is suitable for transmitting over the network or storing in file or database is called Serialization.

In the case of JSON, when we serializing objects, we essentially convert a Python object into a JSON string and deserialization builds up the Python object from its JSON string representation.

Question 351

What is Deserialization?

Answer 351

Deserialization: It is the reverse of serialization. It converts the special format returned by the serialization back into a usable object.

In the case of JSON, when we serializing objects, we essentially convert a Python object into a JSON string and deserialization builds up the Python object from its JSON string representation.

Question 352

Python provides a built-in module called ______ for serializing and deserializing objects

Answer 352

json

The json module mainly provides the following functions for serializing and deserializing:
1. dump(obj, fileobj)
2. dumps(obj)
3. load(fileobj)
4. loads(s)

Question 353

json module:
dump()

Answer 353

The dump() function is used to serialize data. It takes a Python object, serializes it and writes the output (which is a JSON string) to a file like object.

e.g

import json

python object
person = {
‘first_name’: “John”,
“isAlive”: True,
“age”: 27,
“address”: {
“streetAddress”: “21 2nd Street”,
“city”: “New York”,
“state”: “NY”,
“postalCode”: “10021-3100”
},
“hasMortgage”: None
}

writing JSON object
with open(‘person.json’, ‘w’) as f:
json.dump(person, f)

Question 354

Python Type to JSON Type conversion examples

Answer 354

dict - object
list, tuple - array
int - number
float - number
str - string
True - true
False - false
None - null

Question 355

json module:
load()

Answer 355

The load() function deserializes the JSON object from the file like object and returns it.

e.g
with open(‘person.json’, ‘r’) as f:
person = json.load(f)

Question 356

json module:
dumps()

Answer 356

python object

The dumps() function works exactly like dump() but instead of sending the output to a file-like object, it returns the output as a string.

e.g
# python object
person = {
‘first_name’: “John”,
“isAlive”: True,
“age”: 27,
“address”: {
“streetAddress”: “21 2nd Street”,
“city”: “New York”,
“state”: “NY”,
“postalCode”: “10021-3100”
},
“hasMortgage”: None
}

data = json.dumps(person)

> data
‘{“hasMortgage”: null, “isAlive”: true, “age”: 27, “address”: {“state”: “NY”, “streetAddress”: “21 2nd Street”, “city”: “New York”, “postalCode”: “10021-3100”}, “first_name”: “John”}’

Question 357

json module:
loads()

Answer 357

loads() function is as same as load() but instead of deserializing the JSON string from a file, it deserializes from a string.

e.g
# json shown as string
> data
‘{“hasMortgage”: null, “isAlive”: true, “age”: 27, “address”: {“state”: “NY”, “streetAddress”: “21 2nd Street”, “city”: “New York”, “postalCode”: “10021-3100”}, “first_name”: “John”}’

deserialize from string
person = json.loads(data)

Question 358

List some optional keyword arguments that can be passed to the dumps or dump() function to customize the Serializer.

Answer 358

indent, sort_keys, skipkeys, separators

Question 359

dump() function optional arguments:
indent

Answer 359

indent - a positive integer which determines the amount of indentation of key-value pairs at each level. The indent arguments come in handy to prettify the output if you have deeply nested data structures. The default value of indent is None.

e.g
print(json.dumps(person, indent=4)) # with 4 levels of indentation

Question 360

dump() function optional arguments:
sort_keys

Answer 360

A boolean flag, if set to True returns a JSON string ordered by keys, instead of being randomly ordered. Its default value is False.

e.g
# print JSON string in order by keys
print(json.dumps(person, indent=4, sort_keys=True))

Question 361

dump() function optional arguments:
skipkeys

Answer 361

JSON format expects the keys to be a string, if you try to use a type which can’t be converted to a string (like tuple) then a TypeError exception will be raised. To prevent the exception from being raised and skip the non-string keys set the skipkeys argument to True.

e.g
data = {‘one’: 1, ‘two’: 2, (1,2): 3}

json.dumps(data, indent=4)

> Traceback (most recent call last):
…
TypeError: key (1, 2) is not a string

To prevent the exception from being raised and skip the non-string keys use the skipkeys argument

print(json.dumps(data, indent=4, skipkeys=True))

Question 362

dump() function optional arguments:
separators

Answer 362

It refers to a tuple of the form (item_separator, key_separator). The item_separator is a string which is used to separate items in a list. The key_separator is also a string and is used to separate keys and values in a dictionary. By default, the separators set to (‘,’, ‘: ‘).

e.g
#The following example changes and item_separator and key_separator to pipe (|) and dash (-) characters respectively:

print(json.dumps(employee, indent=4, skipkeys=True, separators=(‘|’, ‘-‘)))

Question 363

To serialize custom objects or built-in types, we have to ________________________________________________

Answer 363

create our own serialization function:

def serialize_objects(obj):
… if isinstance(obj, datetime):
… return {
… ‘__class__’: datetime.__name__,
… ‘__value__’: str(obj)
… }
… raise TypeError(str(obj) + ‘ is not JSON serializable’)

The function takes a single argument named obj, check the type of the object using the isinstance() function.
Next, it creates a dictionary with two keys: __class__ and __value__.

The __class__ key stores the original name of the class and will be used to deserialize the data.

The __value__ key stores the value of the object, in this case, we are simply converting datetime.datetime object to its string representation using the built-in str() function.

In the last line, we raise TypeError exception. This is necessary otherwise our serialization function wouldn’t report errors for objects it can’t serialize.

We can pass custom serialization function to dumps() or dump() using the default keyword argument

e.g
# python object:
employee = {
‘first_name’: “Mike”,
“designation”: ‘Manager’,
“doj”: datetime(year=2016, month=5, day=2), # date of joining
}

emp_json = json.dumps(employee, indent=4, default=serialize_objects)

Question 364

To deserialize custom objects or built-in types, we have to ________________________________________________

Answer 364

create our own deserialization function:

def deserialize_objects(obj):
… if ‘__class__’ in obj:
… if obj[‘__class__’] == ‘datetime’:
… return datetime.strptime(obj[‘__value__’], “%Y-%m-%d %H:%M:%S”)
… # if obj[‘__class__’] == ‘Employee’:
… # return Employee(obj[‘__value__’])
… return obj

Question 365

It turns out that the_____ module is not the only way to serialize data. Python provides another module called ______ to serialize and deserialize data.

Answer 365

json, pickle

Question 366

4 main differences between the json and pickle module

Answer 366

1. Pickle is Python specific
2. Pickle serializes to binary
3. Pickle allows to serialize custom objects
4. Pickle is fast

Ad 1. The pickle module is Python-specific which means that once the object is serialized you can’t deserialize it using another language like PHP, Java, Perl etc. If interoperability is what you need stick to the json module.
Ad 2. Unlike json module which serializes objects as human-readable JSON string, the pickle module serializes data in the binary format.
Ad 3. The json module allows us to serialize only the basic Python types (like int, str, dict, list etc.). If you need to serialize custom objects you would have to supply your own serialization function. However, the pickle module works with a wide variety of Python types right out of the box, including the custom objects you define.
Ad 4. Most of the pickle module is coded in C. So it provides a great performance boost while handling large data sets as compared to the json module.

Question 367

How to serialize data with pickle?

Answer 367

First, we open the file in binary mode instead of text mode:

Pickling data is done via the dump() function. It accepts data and a file object. The dump() function then serializes the data and writes it to the file.

e.g

import pickle
f = open(“my_pickle”, “wb”)

Second, the dump() function is able to serialize the data:

pickle.dump(“a string”, f)
f.close()

Question 368

How to deserialize data with pickle?

Answer 368

The load() function takes a file object, reconstruct the objects from the pickled representation, and returns it.

f = open(“my_pickle”, “rb”)
pickle.load(f)
‘a string’

Once you have unpickled the data you can use it like an ordinary Python object.

Question 369

pickle module
dumps()

Answer 369

The dumps() works exactly like dump() but instead of sending the output to a file, it returns the pickled data as a string.

e.g
# python object
employee = {
“first_name”: “Mike”,
“designation”: ‘Manager’,
“doj”: datetime(year=2016, month=5, day=2), # date of joining
… }

pickled_emp = pickle.dumps(employee)

> pickled_emp
b’\x80\x03}q\x00(X\x0b\x00\x00\x00designationq\x01X\x07\x00\x00\x00Managerq\x02X\x03\x00\x00\x00dojq\x03cdatetime\ndatetime\nq\x04C\n\x07\xe0\x05\x02\x00\x00\x00\x00\x00\x00q\x05\x85q\x06Rq\x07X\n\x00\x00\x00first_nameq\x08X\x04\x00\x00\x00Mikeq\tu.’

Question 370

pickle module
loads()

Answer 370

Similarly, the loads() function is same as load(), but instead of reading pickled data from a file, it reads from a string.

e.g
employee = {
“first_name”: “Mike”,
“designation”: ‘Manager’,
“doj”: datetime(year=2016, month=5, day=2), # date of joining
… }

pickled_emp = pickle.dumps(employee)

> pickled_emp
b’\x80\x03}q\x00(X\x0b\x00\x00\x00designationq\x01X\x07\x00\x00\x00Managerq\x02X\x03\x00\x00\x00dojq\x03cdatetime\ndatetime\nq\x04C\n\x07\xe0\x05\x02\x00\x00\x00\x00\x00\x00q\x05\x85q\x06Rq\x07X\n\x00\x00\x00first_nameq\x08X\x04\x00\x00\x00Mikeq\tu.’

pickle.loads(pickled_emp)

> pickled_emp
{‘designation’: ‘Manager’, ‘doj’: datetime.datetime(2016, 5, 2, 0, 0), ‘first_name’: ‘Mike’}

Keep in mind that, when you unpickle data, objects spring into life, so never try to process pickled data from untrusted sources. A malicious user can use such technique to execute arbitrary commands on the system.

Answer 372

Unit testing is the testing of single components of the application to check 1 functionality at a time