Python Functions

Overview

Functions encapsulate logic you want to reuse. Python functions are first-class objects—you can pass them as arguments, return them, and assign them to variables. Use def to define and return to send a value back to the caller.

Syntax / Usage

def greet(name, greeting="Hello"):
    """Return a greeting string."""
    return f"{greeting}, {name}!"

# Positional and keyword arguments
greet("Ada")
greet("Ada", greeting="Hi")

# Arbitrary arguments
def summarize(*args, **kwargs):
    return args, kwargs

summarize(1, 2, key="value")

# Lambda (anonymous one-liners)
double = lambda x: x * 2

Type hints (optional but recommended) document expected types without enforcing them at runtime.

Examples

Calculate order total with tax:

def order_total(subtotal: float, tax_rate: float = 0.08) -> float:
    return round(subtotal * (1 + tax_rate), 2)

order_total(100)       # 108.0
order_total(100, 0.1)  # 110.0

Validate password length:

def is_valid_password(password: str, min_length: int = 8) -> bool:
    return len(password) >= min_length

Common Mistakes

• Using mutable default arguments (def f(items=[]):)—defaults are created once
• Forgetting return yields None implicitly
• Shadowing built-ins like sum or max with parameter names
• Relying on global state instead of passing explicit parameters

See Also

python-variables python-decorators python-modules