Python

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0. Introduction

• We use programming language to communicate with a computer. It is a High-Level Language

• The Python Interpreter converts the Python scripts in Byte Code that then the Python VM runs it

• We need to have Python installed on our system to use it

• Created by Guido Van Rossum in 1991 inspired by the Monty Python’s Flying Circus British Comedy Show

• The python REPL can be run using the terminal which can be opened by typing python in terminal window

• To run a python file: python filename.py

1. Variables & Data Types

• A variable is a container that is stored in the memory

• A variable is also called as an identifier a = 10 here a is an identifier

• Numbers: int , float, complex

• str, bool, None

1. Can’t start with a number and contain any special characters other than _

2. Names must be snake_case

# Store multiple Values
a, b, c, = 1, 2, 3

# Other Naming Conventions
# camelCase
# PascalCase
# kebab-case
# snake_case
# mumblecase

1. Arithmetic Operators: +, -, *, /

2. Assignment Operator: =, +=, -=, etc.

3. Comparison Operators: ==, >, ≥, ≠, etc. (Always a bool)

4. Logical: and, or, not

Types

• The type() function gives us the type

• Type casting means converting from one type to another. int(), float(), str(), bool(), list()

• The input() function helps to take input but as a str

2. Strings

• Sequence of characters enclosed in quotes. Single, Double of Triple Quotes (multiline)

• Strings are immutable

• Strings have index position starting from 0. The length len() still is natural number

• Python also supports negative indices (backwards)

• All string methods gives us a new string they don’t mutate the original

• Comments: either # or """ """ for multiline (3 quotes start/end)

# Multiline Strings
print("""
	Multiple
	Lines
	We can print
""")

print("Hello", end="") # Will not go in new line

# String Slicing
name = "Phleebs"
print(name[start:end:step]) # Inclusive, Exclusive

name.index(el) # Gives index or error
name.find(el) # Index of -1 if not found
'a' in name # bool
name.replace(with, what) # Returns new str

# Create duplicate
new_name = name[:]
new_name[:5] # From starting
new_name[2:] # From 2 all the way upto end
new_name[::2] # Every second
new_name[::-1] # Reversed String

# Concatenation
print("First" + "Second") #FirstSecond

# F Strings
print(f"The sum of 2 and 2 is {2 + 2}") # Interpolation
# Format String
print("This is something {}".format("Hello")) # Old

Methods

string = "Phleebs fugers rovity"

len(str)
string.index(el) # Index or error
string.find(el) # Index or -1
string.replace(with, what)
string.startswith("Ph") # True
string.endswith('yo') # False
string.upper()
string.lower()
string.capitalize()
string.title() # Phleebs Fugers Rovity
string.count('a') # 0
string.strip() # Removes whitespace from both ends

Escape Sequence Characters

• For newline \n, \t, \b, \” (skips)

3. Lists & Tuples

Lists

• List is an ordered container to store values of any data type

• Lists are mutable

fruits = ["🍎", "🍌", "🍊", "🥝", "🍍"]

fruits.append("Chai") # Ends at end
fruits.insert(index, element)
fruits.extend(iterable) # Takes an iterable
fruits.pop() # Removes from end
fruits.pop(index) # Removes at index
fruits.remove(element) # Takes value
fruits.reverse() # Mutates original
fruits.count(el) # Count occurences

# Pokemon List
fruits = ["🍎", "🍌", "🍊", "🥝", "🍍"]
print('-'.join(fruits)) # 🍎-🍌-🍊-🥝-🍍

# List Unpacking
a, b, *c = pokemon (🍎, 🍌, [...])

Comprehension: TODO

• [expression for item in iterable]

• [expression for item in iterable if condition]

Matrix

• 2D or multi-dimensional list

• Used in image processing

matrix = [
	[1, 0, 1],
	[0, 2, 0],
	[1, 0, 1]
]

# Take out the 2 from the matrix
print(matrix[1][1])  # 2

Tuples

• Immutable lists

• Very few methods but very fast and optimal

• Used when we need a collection that is not going to mutate

my_friends = ("Cat", "Kowel", "Dino", "Puppy", "Rabbit")
a = (1,) # Tuples with only one element

# Methods
my_fruits.count("Cat") # 1
my_fruits.index(el)

# Concatenation
new_tuple = first_tuple + second_tuple

# Membership
"Cat" in my_fruits # True

# Unpacking
a, b, *c = my_fruits
print(a, b, c) # "Cat" , "Kowel", [rest]

4. Dictionary & Set

• Dictionary are key value pairs

• They are also called as Objects in JS

• The data we get from an API is in JSON format which is similar to a dict

• It is unordered (now ordered), mutable and indexed, can’t contain duplicate keys

friends_loan = {
	"Alok": 100,
	"Shubham": 20,
	"Rohan": 10,
	"Sir": 60
}

# Methods
friends_loan.keys()
friends_loan.values()
friends_loan.items() # list of tuples

 # Updates otherwise adds them
friends_loan.update({"Alok": 120, "Smith": 10})

# Will give error
friends_loan["Ajay"]

# None, if we use bracket
# notation we get an error
friends_loan.get("Ajay")

# friends_loan.pop('Sir', none)
del friends_loan["Sir"]

Deep vs Shallow Copy

student_copy = student.copy() # shallow copy

Sets

• Is a collection of non-repetitive element

• It is unordered, unindexed

• A set can’t contain immutable like a list or a dict

 

first = {1, 2, 3, 4, 5, "Yo"}
second = {"Yo", 5, 7, 8, 9}

first.add(el) # Add item
first.remove(el) # Removes element
first.discard(el) # safe remove
first.pop() # Removes any random 😹
first.intersection(second) # "Yo", 5
first.union(second) # All
first.clear() # Empties the set
first.difference(second) # Only unique ones
first.issubset, first.issuperset
first.isdisjoing(second) # True when they are complete distinct

# Creating an empty set
our_set = set()

5. Conditional Expression

• Also called as control flow allows us to make decisions in our code

• They are used to guide logical flow of our program

if condition:
	pass # null operation
elif condition:
	pass
else:
	pass

Operators

# Relational Operators
==
>=
<=

# Logical Operators
and # both needs to be True
or # any one needs to be True
not # flips the value

Truthy & Falsey

• Some values are false and others are true

• We can check them using the bool() function

• None, False, 0, 0.0, 0j and empty list, tuple, dict are false

Ternary Operator

• It is a short hand of a simple if else block

• This is an expression meaning that we can use inside a formatted string

age = 18
license = 'Approve' if age >= 18 else "Reject"

6. Loops

• Used to iterate or repeat something

• There are two types of loops: while and for

While

• While is when we don’t know prior how many iterations to perform: while True:

• While loops are the most common to create an infinite loop to occur

• Write a program to print numbers from 1 to 100 using a while loop

• Use a while loop to print all the elements of a list

• We can also append an else clause with loops

# Else will run after the final iteration
i = 1
while i <= 10:
	print("Working")
	i += 1
else:
	print("Done")

For

• We can use it for a pre-defined loop: for i in range(20) 20 times loop

• To iterate over a list or any iterable for i in iterable

• We can also add an else clause with a for that will run when the loop will end

numbers = [1, 2, 3, 4, 5]
 
for number in numbers:
	print(number)
else:
	print("The execution has completed!")

• The break, continue and pass keyword. The break keyword will break the loop execution, the continue will skip and pass is used as an empty body placeholder. The pass keyword instructs to do nothing

Using Enums

• They are helpful when we need both the index and the element

for i, char in enumerate("Chai"):
	print(i, char)
	
# 0 C
# 1 h
# 2 a
# 3 i

7. Functions

• Block of code that can be reused. It helps in achieving DRY code

• A function is like a machine that takes in some input (arguments) and outputs (returns) something

• We define a function and then use it (invoke or call)

• The code after the return keyword doesn’t run

• There are two types of functions: built-in functions (like print(), type()) and user defined custom functions using the def keyword

• Pure functions don’t create side effects meaning that they don’t mutate variables present in their outside scope

def greet(name = "User"): 
	print(f"Hello {name}")

Default Parameters & Keyword Arguments

• The default way of taking and receiving input is called positional parameters

• Docstring is a mechanism to write what the function does

# Giving Default Parems
def sayHi(name="User"):
	print(f"Hello {name}")

# Passing Arguments as Keywords
def greet(first, second):
    print(f"It's nice to meet you {first} {second}")
    
greet(second='Rishabh', first='Dora') # Keyword Arguments


# Docstring
def kill():
    """
    This function kills people
    who doesn't listen to Sidhu Moose Wala
    """
    print("Listen to Sidhu Moose Wala")
    
help(kill) # Will show up the docstring

Recursion

• Meaning a function that keeps on calling itself

• There are several times when we need to solve a problem using recursion

• It is a famous technique of solving a branch of computer and mathematical problems

• Improper use can also cause an issue of infinite loop

• Solve the factorial problem using recursion

• Task: Create a function to find sum of first n natural numbers using recursion

# Solving Factorial Problem Using Recursion
 
def find_factorial(n):
    if (n == 0) or (n == 1):
        return 1
    return n * find_factorial(n-1)

x = find_factorial(5)
print(x)

Scope

• Scope defines visibility of elements

• The default scope is called the Global Scope

# An impure function as it modifies a global variable
x = 100
def first():
    global x # Will reference the outside scope
    x = 200
    print(x)
    
first()
print(x)

*args N Positional Arguments

• With the help of *args we can take any number of positional arguments inside a function

• Define a function that takes any number of inputs of numbers and returns the maximum

def find_max(*nums):
    max = None
    for n in nums:
        if max == None or max < n:
            max = n
    return max

x = find_max(1, 2000, 3, 4)
print(x) 

8. Modules, Comments & PIP

• A module is a file containing code written by somebody else. We can use pip to install modules. For example: requests, flask, django and to install: pip install flask

• We either have built in modules (os) or external modules

• Some common packages are: os, requests, dotenv

• The Python Package Index includes all the packages we can use such as requests, etc.

# Print the contents of the directory
import os
directory_path = "/"
contents = os.listdir(directory_path)
print(contents)

9. File Operations