Python Functions: Everything You Need to Know for Your Exam

Defining and calling functions

A function is defined with `def`, accepts parameters, and returns a value with `return`. If there's no `return` statement, the function returns `None`.

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

result = greet("Alice")  # "Hello, Alice"

def do_nothing():
    pass

print(do_nothing())  # None

Default arguments — and the mutable default trap

Default argument values are evaluated once when the function is defined, not each time it's called. This creates a well-known trap with mutable defaults like lists.

# TRAP — default list is shared across all calls
def add_item(item, lst=[]):
    lst.append(item)
    return lst

print(add_item(1))  # [1]
print(add_item(2))  # [1, 2] — NOT [2]

# FIX — use None as default
def add_item_safe(item, lst=None):
    if lst is None:
        lst = []
    lst.append(item)
    return lst

*args and **kwargs

*args collects extra positional arguments into a tuple. **kwargs collects extra keyword arguments into a dictionary. Both names are conventions — only the * and ** matter.

def total(*args):
    return sum(args)

print(total(1, 2, 3))  # 6

def profile(**kwargs):
    for key, value in kwargs.items():
        print(f"{key}: {value}")

profile(name="Alice", age=30)
# name: Alice
# age: 30

Argument order matters

When mixing argument types, they must appear in this order: positional → *args → keyword-only → **kwargs. Getting this wrong causes a SyntaxError.

def example(a, b, *args, keyword_only, **kwargs):
    pass

# Calling it correctly:
example(1, 2, 3, 4, keyword_only="required", extra="ok")

Lambda functions

A lambda is a one-line anonymous function. It can take any number of arguments but only a single expression — no statements, no assignments.

double = lambda x: x * 2
print(double(5))  # 10

# Often used with sorted(), filter(), map()
names = ["Charlie", "Alice", "Bob"]
sorted_names = sorted(names, key=lambda n: len(n))
# ["Bob", "Alice", "Charlie"]

Exam tip

The most common Python functions question involves mutable default arguments. If you see `def f(x, lst=[])` in an exam question, the answer almost always involves shared state across calls.