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Defining Functions, Parameters & Return Values

5 min read

You'll learn to

-Define a function with `def` and call it with positional arguments
-Understand what a function returns when it has no explicit `return`
-Return multiple values from a single function using a tuple

Every program you have written so far has been a single, flat sequence of statements. Functions are the first tool for breaking that sequence into named, reusable pieces. Every Forge Algorithm Lab level you will eventually submit is, at its core, one or more functions (or methods, which are just functions that live inside a class) that the evaluator calls with specific inputs and checks the output of. Getting comfortable with parameters and return values now pays off immediately.

Defining and Calling a Function

A function is defined with def, a name, a parenthesized list of parameters, and a colon, followed by an indented body. The same indentation rules that govern if blocks and loops apply here.

A simple function with two parameters

def add(a, b):
    total = a + b
    return total

result = add(3, 4)
print(result)  # 7

a and b are parameters, placeholders in the definition. 3 and 4 are arguments, the actual values supplied at the call site. This distinction rarely matters in casual conversation, but it becomes precise and important once default arguments and keyword arguments enter the picture in the next chapter.

What Happens Without a Return Statement

A function does not have to return anything explicitly. If execution reaches the end of the function body without hitting a return statement, or hits a bare return with no value, Python returns None. This is a frequent source of confusion: calling print() inside a function shows output on the screen, but it does not hand a value back to the caller.

A common mistake: printing instead of returning

def double_wrong(x):
    print(x * 2)   # displays the value, but returns nothing

def double_right(x):
    return x * 2   # hands the value back to the caller

y = double_wrong(5)   # y is None - "10" was printed, but not returned
z = double_right(5)   # z is 10
print(y, z)  # None 10

This is one of the single most common bugs in beginner Python code, and it shows up constantly in graded lab submissions: a function that print()s the right-looking answer but returns None, so any code (or evaluator) that calls it and checks the return value fails silently.

Returning Multiple Values

Python lets a function return more than one value by packing them into a tuple. This uses the same tuple-unpacking syntax you already know from earlier chapters. return builds the tuple, and the caller can unpack it directly into separate names.

Returning a tuple of values

def min_max(numbers):
    return min(numbers), max(numbers)

lowest, highest = min_max([4, 1, 9, 2, 7])
print(lowest, highest)  # 1 9

# Or keep it as a single tuple if you don't need to unpack immediately:
result = min_max([4, 1, 9, 2, 7])
print(result)       # (1, 9)
print(result[0])    # 1

-return a, b is really return (a, b). The parentheses are optional for tuples.
-A function can only have one return statement execute per call. Once a return runs, the function exits immediately, even mid-loop.
-Calling return with no value, or omitting return entirely, both produce None.

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Slicing, Iteration & Comprehensions Default Arguments, Keyword Arguments & *args/**kwargs

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