Learn Python – Beginner Guide | Algo Infinity Verse

01

Introduction to Python

Python is a high-level, interpreted programming language known for its simple, readable syntax. It was created by Guido van Rossum and first released in 1991. Today it powers everything from web apps to AI research.

Why Learn Python?

Easy to learn – clean, English-like syntax
Versatile – web dev, data science, automation, AI/ML
Massive community – tons of libraries and resources
In-demand – one of the most sought-after languages in jobs

Your very first Python program is the classic "Hello, World!" — just one line:

Python

print('Hello, World!')

Output

Hello, World!

💡

Python uses indentation (whitespace) to define code blocks instead of curly braces {}. This enforces clean, readable code.

02

Installation & Setup

Follow the steps below to install Python on your machine. Most systems come with Python pre-installed, but it's best to verify you have a recent version (3.9+).

macOS

Install via Homebrew (recommended):

Bash

brew install python3

Windows

Download the installer from python.org/downloads and check "Add Python to PATH" during installation.

Linux

Most distributions include Python. Install or update via your package manager:

Bash

sudo apt update && sudo apt install python3

✅

Verify your installation by running python3 --version in your terminal.

Bash

python3 --version
# Expected: Python 3.11.x (or newer)

You can also use the Python REPL (Read-Eval-Print Loop) by simply typing python3 in your terminal to start writing code interactively.

03

Variables & Data Types

Variables are named containers that store data. In Python, you don't need to declare a type — the interpreter figures it out automatically.

Python

name = "Alice"       # str
age = 25              # int
height = 5.7          # float
is_student = True     # bool

print(type(name))
print(type(age))

Output

<class 'str'>
<class 'int'>

Type	Example	Description
`int`	`42`	Whole numbers
`float`	`3.14`	Decimal numbers
`str`	`"hello"`	Text (strings)
`bool`	`True / False`	Boolean values
`list`	`[1, 2, 3]`	Ordered, mutable collection
`dict`	`{"key": "val"}`	Key-value pairs

💡

Use type() to check a variable's type, and len() to get the length of a string, list, or dictionary.

Practice: Variables & Data Types

Create variables of each basic type (int, float, str, bool), then print their values and types.

Create an integer variable score with value 95
Create a float variable gpa with value 3.85
Create a string variable city with value "New York"
Create a boolean passed set to True
Print each variable and its type()

Python

score = 95
gpa = 3.85
city = "New York"
passed = True

print(f"Score: {score} ({type(score).__name__})")
print(f"GPA: {gpa} ({type(gpa).__name__})")
print(f"City: {city} ({type(city).__name__})")
print(f"Passed: {passed} ({type(passed).__name__})")

04

Input & Output

Every interactive program needs to read input and display output. Python provides input() and print() for this.

print() — Displaying Output

The print() function outputs text to the console. You can separate values with sep and end with end.

Python

name = "Alice"
age = 25

# f-strings (formatted string literals)
print(f"My name is {name} and I am {age} years old.")

# Multiple values
print("Name:", name, "| Age:", age)

Output

My name is Alice and I am 25 years old.
Name: Alice | Age: 25

input() — Reading User Input

The input() function pauses the program and waits for the user to type something. It always returns a string.

Python

name = input("What is your name? ")
print(f"Hello, {name}! Welcome!")

# Convert input to integer
age = int(input("How old are you? "))
print(f"In 10 years you will be {age + 10}.")

⚠️

input() always returns a str. Wrap it with int() or float() to use it as a number.

Practice: Input & Output

Build a simple greeting program:

Ask the user for their name using input()
Ask for their favorite color
Print a message: "Hello {name}, your favorite color is {color}!"

Python

name = input("Enter your name: ")
color = input("Enter your favorite color: ")
print(f"Hello {name}, your favorite color is {color}!")

05

Conditional Statements

Conditionals let your program make decisions. Python uses if, elif, and else to branch execution based on conditions.

Python

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

print(f"Score: {score} → Grade: {grade}")

Output

Score: 85 → Grade: B

Comparison Operators

== equal to
!= not equal to
>, <, >=, <= — greater/less than
and, or, not — logical operators

Python

age = 20
has_id = True

if age >= 18 and has_id:
    print("Entry allowed")
else:
    print("Entry denied")

Practice: Conditionals

Write a program that:

Reads a number from the user
Prints "Positive" if it's greater than 0
Prints "Negative" if it's less than 0
Prints "Zero" if it's exactly 0

Python

num = float(input("Enter a number: "))

if num > 0:
    print("Positive")
elif num < 0:
    print("Negative")
else:
    print("Zero")

06

Loops

Loops let you repeat a block of code multiple times. Python has two main loop types: for and while.

for Loop — Iterating Over a Sequence

Use for when you know how many times to loop or are iterating over a collection.

Python

# Iterate over a range
for i in range(5):
    print(f"Count: {i}")

# Iterate over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

Output

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4
apple
banana
cherry

while Loop — Repeating Until a Condition is False

Use while when you don't know the exact number of iterations.

Python

count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1

⚠️

Be careful with while loops — always make sure the condition eventually becomes False to avoid infinite loops.

Python

# Break and Continue
for i in range(10):
    if i == 3:
        continue    # skip 3
    if i == 7:
        break       # stop at 7
    print(i)

Output

Practice: Loops

Write two programs:

Use a for loop to print all even numbers from 1 to 20
Use a while loop to find the sum of numbers from 1 to 100

Python

# Even numbers 1-20
for i in range(1, 21):
    if i % 2 == 0:
        print(i)

# Sum of 1 to 100
total = 0
num = 1
while num <= 100:
    total += num
    num += 1
print(f"Sum: {total}")

07

Functions

Functions are reusable blocks of code that perform a specific task. They help you avoid repetition and keep your code organized.

Python

def greet(name):
    """Greet a user by name."""
    return f"Hello, {name}!"

# Call the function
message = greet("Alice")
print(message)

Output

Hello, Alice!

Default Parameters & Multiple Returns

You can set default values for parameters and return multiple values using tuples.

Python

def calculate(a, b, operation="add"):
    if operation == "add":
        return a + b
    elif operation == "subtract":
        return a - b
    elif operation == "multiply":
        return a * b

print(calculate(10, 5))              # 15 (default: add)
print(calculate(10, 5, "multiply"))  # 50

def min_max(numbers):
    return min(numbers), max(numbers)

lo, hi = min_max([3, 1, 7, 2, 9])
print(f"Min: {lo}, Max: {hi}")

Output

15
50
Min: 1, Max: 9

Practice: Functions

Write a function is_even(n) that returns True if n is even, False otherwise
Write a function factorial(n) that returns the factorial of n
Write a function reverse_string(s) that returns the reversed string

Python

def is_even(n):
    return n % 2 == 0

def factorial(n):
    if n <= 1:
        return 1
    return n * factorial(n - 1)

def reverse_string(s):
    return s[::-1]

print(is_even(4))        # True
print(factorial(5))      # 120
print(reverse_string("hello"))  # olleh

08

Lists & Dictionaries

Lists and dictionaries are Python's most commonly used data structures. Lists hold ordered sequences; dictionaries store key-value pairs.

Lists — Ordered & Mutable

Lists can hold any data type and can be modified after creation.

Python

fruits = ["apple", "banana", "cherry"]

# Access
print(fruits[0])       # apple
print(fruits[-1])      # cherry

# Modify
fruits.append("date")
fruits.insert(1, "blueberry")
fruits.remove("banana")

print(fruits)
print(f"Length: {len(fruits)}")

Output

apple
cherry
['apple', 'blueberry', 'cherry', 'date']
Length: 4

List Slicing

Slice syntax: list[start:stop:step] — extract a portion of a list without modifying the original.

Python

nums = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

print(nums[2:5])     # [2, 3, 4]
print(nums[:4])      # [0, 1, 2, 3]
print(nums[::2])     # [0, 2, 4, 6, 8]
print(nums[::-1])    # [9, 8, 7, ... 0] (reversed)

Dictionaries — Key-Value Pairs

Dictionaries store data as key: value pairs. Keys must be immutable (strings, numbers, tuples).

Python

person = {
    "name": "Alice",
    "age": 25,
    "city": "New York"
}

# Access values
print(person["name"])         # Alice
print(person.get("age", 0))   # 25

# Add / update
person["email"] = "alice@mail.com"
person["age"] = 26

# Iterate
for key, value in person.items():
    print(f"{key}: {value}")

Output

Alice
25
name: Alice
age: 26
city: New York
email: alice@mail.com

Practice: Lists & Dictionaries

Create a list of 5 numbers and print their sum and average
Create a dictionary representing a book (title, author, pages, genre) and print each field
Write a function that takes a list and returns a dictionary with each element as a key and its count as the value

Python

# Sum and average
nums = [10, 20, 30, 40, 50]
total = sum(nums)
avg = total / len(nums)
print(f"Sum: {total}, Average: {avg}")

# Book dictionary
book = {
    "title": "1984",
    "author": "George Orwell",
    "pages": 328,
    "genre": "Dystopian"
}
for key, value in book.items():
    print(f"{key}: {value}")

# Count elements
def count_elements(lst):
    counts = {}
    for item in lst:
        counts[item] = counts.get(item, 0) + 1
    return counts

print(count_elements(["a", "b", "a", "c", "b", "a"]))

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🐍 Learn Python

Introduction to Python

Why Learn Python?

Installation & Setup

macOS

Windows

Linux

Variables & Data Types

Input & Output

print() — Displaying Output

input() — Reading User Input

Conditional Statements

Comparison Operators

Loops

for Loop — Iterating Over a Sequence

while Loop — Repeating Until a Condition is False

Functions

Default Parameters & Multiple Returns

Lists & Dictionaries

Lists — Ordered & Mutable

List Slicing

Dictionaries — Key-Value Pairs