Python Functions — A Practical Beginner's Guide

A function in Python is a reusable block of code with a name. It's the single most important building block once your scripts grow past 20 lines.

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

message = greet("Alice")
print(message)

The def keyword defines a function. The colon and indent mark its body. return sends a value back to the caller. A function without return returns None.

Naming and calling

Names use snake_case by convention. Call a function by writing its name with parentheses, even if it takes no arguments: greet(), not just greet.

Parameters vs arguments

These two words are often confused but mean different things:

• Parameters are the names in the definition: def greet(name): — name is a parameter.
• Arguments are the values you pass: greet("Alice") — "Alice" is an argument.

Default arguments

def discount(price, pct=10):
    return price * (1 - pct/100)

discount(100)         # 90.0 — uses the default
discount(100, 25)     # 75.0 — overrides the default

Keyword arguments

When a function has more than 2 or 3 parameters, calling by keyword is clearer:

send_email(to="alice@example.com", subject="Hi", body="...", attachments=[])

Self-documenting at the call site.

Type hints (modern Python — use them)

Type hints don't change runtime behavior, but they help editors, linters, and other humans:

def add(a: int, b: int) -> int:
    return a + b

Add them once you're past the throwaway-script stage. They pay off immensely.

Docstrings

A triple-quoted string right after def becomes the function's documentation:

def discount(price, pct=10):
    """Return price after applying pct% discount.

    pct is an integer percentage. Use 0 for no discount.
    """
    return price * (1 - pct/100)

help(discount) prints the docstring. So does your IDE's hover.

The mutable default argument trap

The single most-asked Python interview question.

def add_to_basket(item, basket=[]):    # WRONG
    basket.append(item)
    return basket

The default list is created once when the function is defined, then reused across every call. Two callers share the same list. Catastrophic bug.

Fix:

def add_to_basket(item, basket=None):
    if basket is None:
        basket = []
    basket.append(item)
    return basket

Rule: never use a mutable default (list, dict, set). Use None and check.

Where this fits in the 7-Day Python Sprint

Functions are introduced on Day 3 — alongside *args, **kwargs, lambdas, and map / filter / sorted. By the end of Day 3 your CLI calculator from Day 2 gets refactored into a clean function library.