🐍 Python Basics Cheatsheet¶

🧠 What Python Is¶

Python is a high-level, interpreted programming language known for readability and simplicity. It focuses on clarity over cleverness. You write .py files and run them directly — no manual compilation needed.

It’s widely used for web backends (Django, Flask, FastAPI), automation, data science, AI/ML, and scripting.

python --version       # check Python version
python main.py         # run script

⚙️ Setup and Environment¶

# Create and activate virtual environment
python -m venv .venv
source .venv/bin/activate      # Linux/Mac
# .venv\Scripts\activate       # Windows

# Install packages
pip install requests

Use venv to isolate dependencies per project. This keeps versions clean and avoids global conflicts.

🧱 Python Basics¶

Variables & Data Types¶

No type declarations — Python is dynamically typed.

x = 42
pi = 3.14
name = "Edgaras"
is_ready = True

Type	Example	Description
`int`	`10`	Whole number
`float`	`3.14`	Decimal number
`str`	`"hello"`	Text
`bool`	`True` / `False`	Logical value
`list`	`[1, 2, 3]`	Ordered, mutable
`tuple`	`(1, 2, 3)`	Ordered, immutable
`set`	`{1, 2, 3}`	Unique, unordered
`dict`	`{"a": 1}`	Key-value pairs

Printing and Comments¶

print("Hello, world!")    # output text

# Single-line comment
"""
Multi-line
comment
"""

🔁 Control Flow¶

if x > 10:
    print("Big")
elif x > 5:
    print("Medium")
else:
    print("Small")

for fruit in ["apple", "banana"]:
    print(fruit)

while x < 5:
    x += 1

Use indentation instead of braces — Python uses whitespace to define scope.

🔧 Functions¶

Functions are defined with def, return values with return.

def greet(name="friend"):
    return f"Hello, {name}!"

print(greet("Edgaras"))

Short functions (lambdas):

add = lambda a, b: a + b

🧩 Collections in Action¶

Lists, sets, tuples, and dicts are the backbone of Python.

# List
nums = [1, 2, 3]
nums.append(4)

# Tuple
coords = (10, 20)

# Set
unique = {1, 2, 2, 3}  # {1, 2, 3}

# Dict
user = {"name": "Alice", "age": 25}
print(user["name"])

List comprehension (Pythonic pattern):

squares = [x*x for x in range(5)]  # [0, 1, 4, 9, 16]

📁 Files and Paths¶

with open("data.txt", "r") as f:
    text = f.read()

with open("output.txt", "w") as f:
    f.write("Hello file!")

Modern way:

from pathlib import Path
data = Path("data.txt").read_text()

🚨 Errors and Exceptions¶

Handle problems gracefully:

try:
    risky()
except ValueError as e:
    print("Error:", e)
finally:
    print("Cleanup done")

Raise your own error:

raise Exception("Something went wrong")

🧱 Classes and Objects¶

Python supports object-oriented programming, but doesn’t force it.

class Dog:
    def __init__(self, name):
        self.name = name

    def bark(self):
        print(f"{self.name} says woof!")

dog = Dog("Max")
dog.bark()

Everything is an object — even functions, classes, and modules.

📦 Modules & Imports¶

Split code across files and reuse it.

import math
from datetime import date

print(math.sqrt(16))
print(date.today())

Run logic only when executed directly:

if __name__ == "__main__":
    main()

✨ Modern Python Features¶

# F-strings (formatted strings)
user = "Alice"
print(f"Hi, {user}!")

# Type hints (optional, for clarity)
def add(x: int, y: int) -> int:
    return x + y

# List comprehension
squares = [n*n for n in range(5)]

# Pattern matching (Python 3.10+)
match command:
    case "start":
        print("Running...")
    case "stop":
        print("Stopped")
    case _:
        print("Unknown")

🧰 Common Tools¶

Tool	Purpose
`pip`	Install packages
`venv`	Virtual environments
`pytest`	Testing
`black`, `flake8`	Code formatting & linting
`jupyter`	Interactive notebooks
`pathlib`	Modern path handling
`requests`	Simple HTTP client

🧭 The Python Mindset¶

Prefer readable over clever.
Use built-ins before reinventing logic.
Code should explain itself — less ceremony, more clarity.
Experiment freely in the REPL (python in terminal).

“Simple is better than complex.” “Readability counts.” — The Zen of Python (import this)