Functions

Python · Reference cheat sheet

📋 Overview

Functions are first-class callables defined with def. They encapsulate behavior, take parameters, and optionally return a value (None if omitted). Prefer small, typed functions with clear names over deep nesting.

🔧 Core concepts

Callables include functions, methods, lambdas, and objects with __call__.

💡 Examples

Typed function with keyword-only args:

def greet(name: str, /, *, shout: bool = False) -> str:
    msg = f"Hello, {name}"
    return msg.upper() if shout else msg

print(greet("Ada"))
print(greet("Ada", shout=True))

Defaults and mutable trap fix:

def append_item(item: str, bucket: list[str] | None = None) -> list[str]:
    if bucket is None:
        bucket = []
    bucket.append(item)
    return bucket

Early return and multiple values:

def parse_int(text: str) -> int | None:
    if not text.strip():
        return None
    try:
        return int(text)
    except ValueError:
        return None

code, label = (200, "ok")  # tuple return pattern

Higher-order:

from collections.abc import Callable

def twice(fn: Callable[[int], int], x: int) -> int:
    return fn(fn(x))

print(twice(lambda n: n + 1, 3))  # 5

⚠️ Pitfalls

• Mutable default arguments (def f(xs=[])) share one list across calls.
• Forgetting return yields None — easy bug in conditionals.
• Annotations are not runtime checks unless you add a validator.
• Closures capture variables by name — see closures.
• Excessive *args/**kwargs hides the real API; prefer explicit params.

🔗 Related

• *args and **kwargs
• Lambda
• Decorators
• Closures
• Typing hints
• Scope / namespaces