Pro lang 2 Flashcards
Assign Multiple Values
VD 1: x, y, z = "Orange", "Banana", "Cherry" print(x) print(y) print(z)
VD 2: x = y = z = "Orange" print(x) print(y) print(z)
Unpack a Collection
If you have a collection of values in a list, tuple etc. Python allows you extract the values into variables. This is called unpacking.
Example
Unpack a list:
fruits = ["apple", "banana", "cherry"] x, y, z = fruits print(x) print(y) print(z)
Global Variables
- Variables that are created outside of a function (as in all of the examples above)
- can be used by everyone, both inside of functions and outside
Create a variable outside of a function, and use it inside the function
Create a variable inside a function, with the same name as the global variable
VD 1
x = “awesome”
def myfunc():
print("Python is " + x)
myfunc()
VD 2:
x = “awesome”
def myfunc():
x = "fantastic"
print("Python is " + x)
myfunc()
print(“Python is “ + x)
The global Keyword
If you use the global keyword, the variable belongs to the global scope:
def myfunc(): global x x = "fantastic"
myfunc()
print(“Python is “ + x)
Type Conversion
x = 1 # int
a = float(x)
print(type(a))
Strings are Arrays
Example
Get the character at position 1 (remember that the first character has the position 0):
a = “Hello, World!”
print(a[1])
String Length
To get the length of a string, use the len() function.
Example The len() function returns the length of a string:
a = “Hello, World!”
print(len(a))
Check String
To check if a certain phrase or character is present in a string, we can use the keyword in.
Example
Check if “free” is present in the following text:
txt = “The best things in life are free!”
print(“free” in txt)
——————————————————–
Print only if “free” is present:
txt = “The best things in life are free!”
if “free” in txt:
print(“Yes, ‘free’ is present.”)
Check if NOT
To check if a certain phrase or character is NOT present in a string, we can use the keyword not in.
Example
Check if “expensive” is NOT present in the following text:
txt = “The best things in life are free!”
print(“expensive” not in txt)
Slicing
You can return a range of characters by using the slice syntax.
Specify the start index and the end index, separated by a colon, to return a part of the string.
Example
Get the characters from position 2 to position 5 (not included):
b = “Hello, World!”
print(b[2:5])/#from start: b[:5]; #to end: b[2:]
Upper Case/ lower case
The upper() method returns the string in upper case:
a = “Hello, World!”
print(a.upper())
Remove Whitespace
Whitespace is the space before and/or after the actual text, and very often you want to remove this space.
Example The strip() method removes any whitespace from the beginning or the end:
a = “ Hello, World! “
print(a.strip()) # returns “Hello, World!
Replace String
Example The replace() method replaces a string with another string:
a = “Hello, World!”
print(a.replace(“H”, “J”))
Split String
The split() method returns a list where the text between the specified separator becomes the list items.
Example The split() method splits the string into substrings if it finds instances of the separator:
a = “Hello, World!”
print(a.split(“,”)) # returns [‘Hello’, ‘ World!’]
String Concatenation
To concatenate, or combine, two strings you can use the + operator.
Example
Merge variable a with variable b into variable c:
a = “Hello”
b = “World”
c = a + b
print(c)
String Format
The format() method takes the passed arguments, formats them, and places them in the string where the placeholders {} are:
Example
Use the format() method to insert numbers into strings:
age = 36
txt = “My name is John, and I am {}”
print(txt.format(age))
Escape Character
To insert characters that are illegal in a string, use an escape character.
An escape character is a backslash \ followed by the character you want to insert.
An example of an illegal character is a double quote inside a string that is surrounded by double quotes:
Escape Characters
Code Result \' Single Quote \\ Backslash \n New Line \r Carriage Return \t Tab \b Backspace \f Form Feed \ooo Octal value \xhh Hex value
STRING METHODS
Method Description
capitalize() Converts the first character to upper case
casefold() Converts string into lower case
center() Returns a centered string
count() Returns the number of times a specified value occurs in a string
encode() Returns an encoded version of the string
endswith() Returns true if the string ends with the specified value
expandtabs() Sets the tab size of the string
find() Searches the string for a specified value and returns the position of where it was found
format() Formats specified values in a string
format_map() Formats specified values in a string
LIST
Lists are used to store multiple items in a single variable.
Lists are one of 4 built-in data types in Python used to store collections of data, the other 3 are Tuple, Set, and Dictionary, all with different qualities and usage.
Lists are created using square brackets:
Example
Create a List:
thislist = [“apple”, “banana”, “cherry”]
print(thislist)
List Length
To determine how many items a list has, use the len() function:
Example
Print the number of items in the list:
thislist = [“apple”, “banana”, “cherry”]
print(len(thislist))
Access Items
List items are indexed and you can access them by referring to the index number:
Example
Print the second item of the list:
thislist = [“apple”, “banana”, “cherry”]
print(thislist[1])/print(thislist[2:5)
Change Item Value
To change the value of a specific item, refer to the index number:
Example
Change the second item:
thislist = [“apple”, “banana”, “cherry”]
thislist[1] = “blackcurrant”/thislist[1:3] = [“blackcurrant”, “watermelon”]
print(thislist)
»[‘apple’, ‘blackcurrant’, ‘cherry’]
Insert Items
To insert a new list item, without replacing any of the existing values, we can use the insert() method.
The insert() method inserts an item at the specified index:
Example
Insert “watermelon” as the third item:
thislist = [“apple”, “banana”, “cherry”]
thislist.insert(2, “watermelon”)
print(thislist)
Append Items
To add an item to the end of the list, use the append() method:
Example
Using the append() method to append an item:
thislist = [“apple”, “banana”, “cherry”]
thislist.append(“orange”)
print(thislist)
Extend List
To append elements from another list to the current list, use the extend() method.
Example
Add the elements of tropical to thislist:
thislist = [“apple”, “banana”, “cherry”]
tropical = [“mango”, “pineapple”, “papaya”]
thislist.extend(tropical)
print(thislist)
Add Any Iterable
The extend() method does not have to append lists, you can add any iterable object (tuples, sets, dictionaries etc.).
Example
Add elements of a tuple to a list:
thislist = [“apple”, “banana”, “cherry”]
thistuple = (“kiwi”, “orange”)
thislist.extend(thistuple)
print(thislist)
Access List Items
thislist = [“apple”, “banana”, “cherry”]
print(thislist[1])/[1:3]
Check if Item Exists
Check if “apple” is present in the list:
thislist = [“apple”, “banana”, “cherry”]
if “apple” in thislist:
print(“Yes, ‘apple’ is in the fruits list”)
Change List Items
Change the second item:
thislist = [“apple”, “banana”, “cherry”]
thislist[1] = “blackcurrant”
print(thislist)
Change a Range of Item Values
Change the values “banana” and “cherry” with the values “blackcurrant” and “watermelon”:
thislist = [“apple”, “banana”, “cherry”, “orange”, “kiwi”, “mango”]
thislist[1:3] = [“blackcurrant”, “watermelon”]
print(thislist)
Insert Items
Insert “watermelon” as the third item:
thislist = [“apple”, “banana”, “cherry”]
thislist.insert(2, “watermelon”)
print(thislist)
Append Items
Using the append() method to append an item:
thislist = [“apple”, “banana”, “cherry”]
thislist.append(“orange”)
print(thislist)
Extend List
To append elements from another list to the current list, use the extend() method.
Example
Add the elements of tropical to thislist:
thislist = [“apple”, “banana”, “cherry”]
tropical = [“mango”, “pineapple”, “papaya”]
thislist.extend(tropical)
print(thislist)
Remove Specified Item
Remove “banana”:
thislist = [“apple”, “banana”, “cherry”]
thislist.remove(“banana”)
print(thislist)
Remove Specified Index
The pop() method removes the specified index.
Example
Remove the second item:
thislist = [“apple”, “banana”, “cherry”]
thislist.pop(1)
print(thislist)
**If you do not specify the index, the pop() method removes the last item
The del keyword
thislist = [“apple” “banana”, “cherry”]
del thislist[0] or del thislist
print(thislist)
Clear the List
The clear() method empties the list.
The list still remains, but it has no content.
Example
Clear the list content:
thislist = [“apple”, “banana”, “cherry”]
thislist.clear()
print(thislist)
Loop Through a List
Print all items in the list, one by one:
thislist = [“apple”, “banana”, “cherry”]
for x in thislist:
print(x)
Loop Through the Index Numbers
You can also loop through the list items by referring to their index number.
Use the range() and len() functions to create a suitable iterable.
Example
Print all items by referring to their index number:
thislist = [“apple”, “banana”, “cherry”]
for i in range(len(thislist)):
print(thislist[i])
Using a While Loop
You can loop through the list items by using a while loop.
Use the len() function to determine the length of the list, then start at 0 and loop your way through the list items by refering to their indexes.
Remember to increase the index by 1 after each iteration.
Example
Print all items, using a while loop to go through all the index numbers
thislist = ["apple", "banana", "cherry"] i = 0 while i < len(thislist): print(thislist[i]) i = i + 1
Looping Using List Comprehension
List Comprehension offers the shortest syntax for looping through lists:
Example
A short hand for loop that will print all items in a list:
thislist = [“apple”, “banana”, “cherry”]
[print(x) for x in thislist]
List Comprehension
List comprehension offers a shorter syntax when you want to create a new list based on the values of an existing list.
Example:
Based on a list of fruits, you want a new list, containing only the fruits with the letter “a” in the name.
Without list comprehension you will have to write a for statement with a conditional test inside:
Example
fruits = [“apple”, “banana”, “cherry”, “kiwi”, “mango”]
newlist = []
for x in fruits:
if “a” in x:
newlist.append(x)
print(newlist)
The Syntax
newlist = [expression for item in iterable if condition == True]
The return value is a new list, leaving the old list unchanged.
Condition
The condition is like a filter that only accepts the items that valuate to True.
Example
Only accept items that are not “apple”:
newlist = [x for x in fruits if x != “apple”]
The condition if x != “apple” will return True for all elements other than “apple”, making the new list contain all fruits except “apple”.
The condition is optional and can be omitted:
Example
With no if statement:
newlist = [x for x in fruits]
Iterable
The iterable can be any iterable object, like a list, tuple, set etc.
Example
You can use the range() function to create an iterable:
newlist = [x for x in range(10)]
Expression
The expression is the current item in the iteration, but it is also the outcome, which you can manipulate before it ends up like a list item in the new list:
Example
Set the values in the new list to upper case:
newlist = [x.upper() for x in fruits]
Sort List Alphanumerically
List objects have a sort() method that will sort the list alphanumerically, ascending, by default:
Example
Sort the list alphabetically:
thislist = [“orange”, “mango”, “kiwi”, “pineapple”, “banana”]
thislist.sort()
print(thislist)
Sort Descending
To sort descending, use the keyword argument reverse = True:
Example
Sort the list descending:
thislist = [“orange”, “mango”, “kiwi”, “pineapple”, “banana”]
thislist.sort(reverse = True)
print(thislist)
Customize Sort Function
You can also customize your own function by using the keyword argument key = function.
The function will return a number that will be used to sort the list (the lowest number first):
Example
Sort the list based on how close the number is to 50:
def myfunc(n): return abs(n - 50)
thislist = [100, 50, 65, 82, 23]
thislist.sort(key = myfunc)
print(thislist)
Case Insensitive Sort
By default the sort() method is case sensitive, resulting in all capital letters being sorted before lower case letters:
Example
Case sensitive sorting can give an unexpected result:
thislist = [“banana”, “Orange”, “Kiwi”, “cherry”]
thislist.sort()
print(thislist)
Reverse Order
What if you want to reverse the order of a list, regardless of the alphabet?
The reverse() method reverses the current sorting order of the elements.
Example
Reverse the order of the list items:
thislist = [“banana”, “Orange”, “Kiwi”, “cherry”]
thislist.reverse()
print(thislist)
Copy a List
You cannot copy a list simply by typing list2 = list1, because: list2 will only be a reference to list1, and changes made in list1 will automatically also be made in list2.
There are ways to make a copy, one way is to use the built-in List method copy().
Example
Make a copy of a list with the copy() method:
thislist = ["apple", "banana", "cherry"] mylist = thislist.copy() print(mylist) ---------- Make a copy of a list with the list() method:
thislist = [“apple”, “banana”, “cherry”]
mylist = list(thislist)
print(mylist)
Join Two Lists
There are several ways to join, or concatenate, two or more lists in Python.
One of the easiest ways are by using the + operator.
Example
Join two list:
list1 = ["a", "b", "c"] list2 = [1, 2, 3]
list3 = list1 + list2
print(list3)
add elements from one list to another list:
Use the extend() method to add list2 at the end of list1:
list1 = ["a", "b" , "c"] list2 = [1, 2, 3]
list1.extend(list2)
print(list1)
List Methods
Method Description
append() Adds an element at the end of the list
clear() Removes all the elements from the list
copy() Returns a copy of the list
count() Returns the number of elements with the specified value
extend() Add the elements of a list (or any iterable), to the end of the current list
index() Returns the index of the first element with the specified value
insert() Adds an element at the specified position
pop() Removes the element at the specified position
remove() Removes the item with the specified value
reverse() Reverses the order of the list
sort() Sorts the list
