13-functions

13 · Functions

Part 3 — Functions & Modules · Estimated time: 35–45 min · Prerequisite: 06 · Conditionals

A function is a named, reusable block of code. You define it once, then call it whenever you need that job done — passing in inputs and getting back an output. Functions keep your code DRY (Don't Repeat Yourself) and let you break a big problem into small, named pieces.

What you'll learn

• Defining a function with def and calling it
• Parameters (inputs) and return (output)
• The difference between a parameter and an argument
• Default values and keyword arguments
• Variable scope (local vs global) and writing docstrings
• Accepting any number of arguments with *args

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1. Defining and calling

def say_hi():
    print("Hi there!")

say_hi()      # call (invoke) the function
say_hi()      # call it again — reuse is the whole point

def starts the definition, the body is indented, and nothing runs until you call it with say_hi().

▶️ Run it: python examples/01_defining_calling.py

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2. Parameters and return

A function can take inputs (parameters) and hand back a value with return.

def square(num):
    return num * num

result = square(5)
print(result)      # 25
print(square(3))   # 9

Parameter vs argument: in def add(a, b), a and b are parameters. In add(2, 4), the 2 and 4 are arguments. Order matters — 2 goes to a.

A function without return hands back None.

▶️ Run it: python examples/02_parameters_and_return.py

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3. Default & keyword arguments

Give a parameter a default so callers can leave it out:

def power(base, exponent=2):
    return base ** exponent

print(power(5))      # 25   (exponent defaults to 2)
print(power(5, 3))   # 125

Pass arguments by name (keyword arguments) for clarity and any order:

print(power(exponent=3, base=2))   # 8

▶️ Run it: python examples/03_default_and_keyword_arguments.py

💡 Try it yourself: write greet(name, greeting="Hello") that returns "Hello, Sam!" by default but can say "Hi, Sam!" when asked.

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4. Scope & docstrings

Variables created inside a function only exist there — that's its local scope. A triple-quoted string just under def is a docstring that documents the function.

def add(x, y):
    """Return the sum of x and y."""
    result = x + y      # 'result' exists only inside add()
    return result

print(add(2, 3))     # 5
print(add.__doc__)   # Return the sum of x and y.
# print(result)      # NameError — 'result' isn't visible out here

▶️ Run it: python examples/04_scope_and_docstrings.py

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5. Any number of arguments: *args

Prefix a parameter with * to gather all the extra arguments into a tuple.

def total(*numbers):
    running = 0
    for n in numbers:
        running += n
    return running

print(total(1, 2, 3))         # 6
print(total(10, 20, 30, 40))  # 100

▶️ Run it: python examples/05_flexible_args.py

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Common mistakes

Mistake	What happens	Fix
return indented inside the loop	exits on the first pass	Indent return to line up with the loop, not inside it.
Forgetting to call with ()	nothing happens (you referenced the function object)	Call it: say_hi().
Expecting a print to be a return	the caller gets None	Use return to send a value back.
Reading a local variable outside	NameError	Return it, or define it in the outer scope.

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Recap / cheat-sheet

def name(param1, param2=default):
    """Docstring describing the function."""
    return value

name(arg1, arg2)          # call with positional args
name(param2=x, param1=y)  # call with keyword args

def f(*args):  ...        # gather extra positional args into a tuple
def f(**kwargs): ...      # gather extra keyword args into a dict

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Exercises

Run a file with python exercises/<file>.py, then compare with the matching file in solutions/.

1. exercises/01_greet.py — write and call a greet(name) function.
2. exercises/02_rectangle_area.py — return a computed value.
3. exercises/03_power_default.py — use a default parameter.

Try each yourself before opening the solution.

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Next → 14 · Lambdas & map/filter (coming soon)