Python Specifics Flashcards
List Comprehensions
An elegant way to create lists in Python in a single line
newlist = [
expression
for item in itterable
if condition == True]
myString=”hello”
myList = [x for x in myString]
RESULT: [‘h’, ‘e’, ‘l’, ‘l’, ‘o’]
// Create a list of even numbers
even = [x for x in range(1,11) if x%2 == 0 ]
fruits = [‘orange’, ‘apple’, ‘grape’, ‘banana’]
newlist = [x if x != “banana” else “orange” for x in fruits]
Walrus Operator
In Python 3.8, you have the WALRUS operator so called as it looks like a walrus := . In plane English, Walrus lets you assigned a variable and check a value at the same time.
With walrus operator
while (data := input(“Enter data (type ‘exit’ to quit): “)) != “exit”:
print(f”Received: {data}”)
Above we set data and checked for data in one line.
Without walrus, we would need to do this:
Without walrus operator
data = input(“Enter data (type ‘exit’ to quit): “)
while data != “exit”:
print(f”Received: {data}”)
data = input(“Enter data (type ‘exit’ to quit): “)
UnPack * & **
In Python, the * and ** operators are used for unpacking when passing arguments to functions or when working with collections such as lists and dictionaries.
def my_function(a, b, c):
print(a, b, c)
my_list = [1, 2, 3]
Unpacking the list into individual arguments
my_function(*my_list)
numbers = [1, 2, 3, 4, 5]
Unpacking the first two elements and the rest into another list
a, b, *rest = numbers
print(a) # Output: 1
print(b) # Output: 2
print(rest) # Output: [3, 4, 5]
For DICTIONARIES, you can unpack the values using **
def my_function(a, b, c):
print(a, b, c)
my_dict = {‘a’: 1, ‘b’: 2, ‘c’: 3}
Unpacking the dictionary into keyword arguments
my_function(**my_dict)
This will set the variables required by the function to the correspopnding values 1,2,3
Function definitions and returns
Unlike Java, Python can return multiple values from a function as a tuple.
def doSomething():
employee=”steve”
salary=50.0
return (employee, salary)
// Two ways to make the call:
result = doSomethin()
(‘steve’, 50). // Return a tuple showing both values
// Or ….
name, newPay = doSomething()
// name is now ‘Steve’, and newPay = 100
// NOTE: if you dont know the parameters a function returns, just use result = doSomething() to see whats returned.
LAMBDA
In Python, a Lambda is just an anonymous function with no name in one line.
lambda argument : expression
Example instead of this function
def add(n1,n2):
return n1+n2
We have
mylambda = lambda n1, n2 : n1+n2
myLambda(5,6). // Returns 11
Lambda function to add two numbers
add = lambda x, y: x + y
Using the lambda function
result = add(5, 3)
print(result) # Output: 8
******
SEE ALSO MAPS & FILTERS WITH LAMBDA
Using MAPS
Map = Takes a function plus an iterable dataset and executes that function for each item.
Example:
def square(num):
return num*num
my_nums=[1,2,3,4,5]
Now we can use map to execute square for every value it my_nums and iterate over the results:
for item in map(square, my_nums):
print (item)
Or you can cast the results to a list using the list function:
myList = list(map(square, my_nums))
Often Map can be used with a LAMBDA in place of a regular function:
numbers = [1, 2, 3, 4, 5]
Using lambda with map to square each number
squared_numbers = list(map(lambda x: x ** 2, numbers))
print(squared_numbers) # Output: [1, 4, 9, 16, 25]
FIlters
FILTER
Similar to Map but function must return boolean. The returned result set will filter out all false values. Filter (function, iterable) - where each element of the iterabel is passed automatically to the ‘function’ which returns either true or false to filter out the data.
Example:
To filter out all even numbers…..
my_data = [1,2,3,4,5,6]
def check_even(num):
return num % 2 == 0
myList = list(filter(check_even, my_data))
[2,4,6]
Lambda = Anonymous Functions , often used with map or filter. Above can be re-written as
myList = list(map(lambda num: num*num, [1,2,3,4,5,6])
[2,4,6]
myReverse = list(map(lambda x: x[::-1], [“steve”, “lilly”, “olivia”]))
[‘evets’, ‘yllil’, ‘aivilo’]
OO IN PYTHON
class MyClassName:
def __init__(self, param1):
self.param1 = param1
def do_some_method(self):
print (self.param1)
abc = MyClassName(“hello”)
abc.do_some_method()
ABSTRACT CLASS IN PYTHON
from abc import ABC, abstractmethod
class Base(ABC):
def \_\_init\_\_(self):
print("Animal Initialised")
@abstractmethod
def foo(self):
Pass
GENERATORS
a generator is a special type of iterable that allows you to iterate over a sequence of values lazily (one at a time), without storing the entire sequence in memory at once. Generators are used to create iterators and are particularly useful when working with large datasets or streams of data because they generate values on the fly.
It uses YIELD to pass control back. each subsequent call to the function continues from where it left off.
You can manually retrieve the next value using the next() function or allow iterrator to do it.
example:
def simple_generator():
for i in range(1, 4):
yield i
——
gen = simple_generator()
Manually retrieve values using next()
print(next(gen)) # Output: 1
print(next(gen)) # Output: 2
print(next(gen)) # Output: 3
or using an interator:
gen = simple_generator()
Iterating over the generator using a for loop
for value in gen:
print(value)
What is PEP8
PEP 8 is the style guide for Python code, promoting readability and consistency. It covers code layout, naming conventions, and best practices. Following PEP 8 helps developers write clear, maintainable code, facilitating collaboration and reducing errors.
== v is
is checks 2 refs reference to same actual object
== can be used to compare values including entire lists
a = [1, 2, 3]
b = [1, 2, 3]
c = a
a == b # True (values are equal)
a is b # False (different objects)
a is c # True (same object)
how to use enumerate in a for loop
in a for loop, we can print the index and the value of an iterable like this:
myList = [a,d,24,”hello”]
for value, e in enumerate(myList):
print (value, e)
This will print
0 a
1 d
2 24
3 “hello”
How do you access a single value in a set ?
you cant.
Sets can be iterarted and you can check if a value is in a set, but you cant access a value from a set direectly.
x = Set(1,2,3)
you can create a set directly using the {} which is usually used for Dictionary.
x = {1,2,3}
This is a SET. Its different to dictionary because it doesn’t use a key/value pair with a colon “:”
example this is a dictionary x = {1:’a’,2:’b’,3:’c’}
how to access keys and values from dictionary
x = {1:’a’,2:’b’,3:’c’}
print (x.keys())
print (x.values())
print(1 in x)
del x[1]
Iterate the Dictionary
for key in x:
print(key)
or to get value and key
for key,value in x.items():
print (key,value)
Functions and multiple returns
python supports functions that can return multiple values.
def myFunc(a,b,c=True):
return a,b
x,y = myFunc(1,2)
It uses a TUPLE to return multiple values.
By specifying a value for a parameter, we make it optional.
A function can even return another function:
def func1(x):
def func2():
print(x)
return func2
a = func1(3)()
This will call the interal func2 with param 3
xargs / kwargs. * and ** (pack/unpack)
- will unpack an input and send it though as seperate arguments
** is exactly the same but will seperate out a dictionary and pass Key (parameter) and Value.
x = [1,2,3,4]
print (*x)
using this with a function to unpack parameters:
def myFunc(a,b,c):
print (a+b+c)
values = (1,2,3)
myFunc(values). // unpacks Tuple and sends 3 params of 1, 2, 3
******
values = {“a”:1, “b”:2, “c”:3}
myFunc(**values)
global and scope
** BAD PRACTICE _ DONT USE IT IDEALLY **
a = ‘hello’
def func():
global a
a = “steve”
print (a)
try / except / throws
try:
a = 5/0
except Exception as e:
print(e)
finally():
print(“This always Runs. Cleanup”)
throw Exception(“its broken”)
****
Python multi threading module
The module is called Threading and supersede the old Thread module
To implement threading , inherit from Thread Class and override run method
from threading Import *
class Example (Thread):
def run(self)
Do stuff
app = Example()
app.start()
