Python 3 Essential Quick Reference & Syntax Cheatsheet

Fast reference guide for Python lists, dictionaries, list comprehensions, slicing, decorators, file I/O, and modern pattern matching.

1. List Comprehensions & Filtering

List and dictionary comprehensions offer a concise syntax when you want to create a new collection based on the values of an existing iterable.

CODE SNIPPET

# Square even numbers
squares = [x**2 for x in range(10) if x % 2 == 0]
# Output: [0, 4, 16, 36, 64]

# Dictionary comprehension
squares_map = {x: x**2 for x in range(5)}
# Output: {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

2. Modern Structural Pattern Matching (Python 3.10+)

Match statements provide expressive pattern matching similar to switch/case in other languages but with destructuring capability.

CODE SNIPPET

def handle_command(command):
    match command.split():
        case ["load", filename]:
            return f"Loading data from {filename}"
        case ["save", filename]:
            return f"Saving data to {filename}"
        case ["quit" | "exit"]:
            return "Exiting runtime"
        case _:
            return "Unknown command"

3. Context Managers & Safe Resource Cleanup

Always wrap network connections, database transactions, and file handles in with blocks to guarantee deterministic cleanup.

CODE SNIPPET

# Reading files automatically closing file handles
with open('dataset.csv', 'r', encoding='utf-8') as f:
    for line in f:
        process(line.strip())

# Custom context manager
from contextlib import contextmanager

@contextmanager
def timer():
    import time
    start = time.perf_counter()
    yield
    print(f"Elapsed: {time.perf_counter() - start:.4f}s")

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