Functions

Functions

What it means

A function is a reusable, named block of code defined with def. It can accept parameters (with optional default values), and returns a value with return (or None implicitly if there’s no return). Functions are the primary tool for avoiding repeated code and giving logic a clear name.

Examples

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

print(greet("Alice"))                # Hello, Alice!
print(greet("Bob", greeting="Hi"))   # Hi, Bob!

def add(*numbers):                   # accepts any number of positional args
    return sum(numbers)

print(add(1, 2, 3, 4))               # 10

def describe(**details):             # accepts any number of keyword args
    for key, value in details.items():
        print(f"{key}: {value}")

describe(name="Alice", age=30)

Common mistake

Using a mutable default argument (like a list or dict) — the default is created once when the function is defined, not each time it’s called, so it silently persists and accumulates state across calls.

def add_item(item, items=[]):     # BUG: same list reused every call
    items.append(item)
    return items

print(add_item("a"))   # ['a']
print(add_item("b"))   # ['a', 'b']  <- unexpected! carried over from last call

# Fixed:
def add_item(item, items=None):
    if items is None:
        items = []
    items.append(item)
    return items

Quick practice

1. What does a function return if it has no explicit return statement?

   Answer

   None.

2. What’s the danger of def f(items=[]):?

   Answer

   The default list is created once at function-definition time and shared across every call that doesn't pass its own items, so mutations accumulate unexpectedly between calls.

3. What’s the difference between *args and **kwargs?

   Answer

   *args collects extra positional arguments into a tuple; **kwargs collects extra keyword arguments into a dict.