Programming Concepts

Programming Essentials~25 min

Functions

A function packages a transformation behind a name, so the code that uses it only needs to know its inputs and outputs, not how it works inside.

By the end of this lesson, you can

  • Write a function with parameters and a return value that packages a small transformation
  • Trace a function call, showing how arguments map to parameters and how the return value flows back to the caller
  • Explain the difference between a function that returns a value and one that only prints

Why it matters

Once a transformation is useful more than once — converting a temperature, calculating a price, validating an answer — copying its code every place it’s needed becomes a maintenance problem: fix a bug in one copy, and every other copy still has it. A function gives a sequence of steps a name, so it can be reused, and so the code that calls it only needs to know what goes in and what comes back out — not how the inside works.

Mental model

A function has an interface: parameters (the named inputs it expects) and a return value (the output it sends back). Everything between them is a detail the caller doesn’t need to think about.

PythonA unit conversion, packaged as a function
def celsius_to_fahrenheit(celsius):
    fahrenheit = celsius * 9 / 5 + 32
    return fahrenheit

print(celsius_to_fahrenheit(0))
print(celsius_to_fahrenheit(100))
Output
32.0
212.0

celsius is a parameter — a placeholder name the function uses for whatever value it’s given. return sends a value back to wherever the function was called, to be used just like any other expression.

Trace it

Tracing a function call means following the value as it moves in through the parameter and back out through return:

Two calls to celsius_to_fahrenheit
Callcelsius (parameter)fahrenheit (inside the function)Returns
celsius_to_fahrenheit(0)00 × 9 / 5 + 32 = 32.032.0
celsius_to_fahrenheit(100)100100 × 9 / 5 + 32 = 212.0212.0

Each call gets its own fresh celsius and fahrenheit — the two calls don’t interfere with each other at all, even though they use the same names.

Check your understanding

What does this print? def double(number): return number * 2 result = double(4) + 1 print(result)
Or reveal the answer without checking

Answer:9
double(4) evaluates to 8 (returned, not printed), and that value is used directly in the expression double(4) + 1, which is 9. result refers to 9.

A function with no return statement...
Or reveal the answer without checking

Answer:Automatically returns None
A function always returns something. If it has no return statement, Python automatically returns None — printing inside a function has no effect on what it returns.

Practice: warm-up

Trace three calls to this function — write down the parameter, what happens inside, and the return value — before revealing the answer.

def area(width, height):
    return width * height

Calls: area(3, 4), area(10, 2), area(5, 5).

Stuck? Reveal one hint at a time.

  1. Hint 1

    Each call gives width and height fresh values — write down what each parameter refers to for that call.

  2. Hint 2

    The function has one line: multiply the two parameters and return the result.

Reveal the trace

Try the problem yourself before reading this. There is often more than one reasonable approach — treat this as one worked example, not the only correct answer.

area(3, 4)   -> width=3,  height=4  -> returns 12
area(10, 2)  -> width=10, height=2  -> returns 20
area(5, 5)   -> width=5,  height=5  -> returns 25

Practice: apply it

This function is supposed to report whether a number is even, but using it always seems to fail. (number % 2 is Python’s remainder operator — it evaluates to the remainder after dividing by 2, so it’s 0 for even numbers and 1 for odd ones.)

def is_even(number):
    if number % 2 == 0:
        print(True)
    else:
        print(False)

if is_even(4):
    print("even!")
else:
    print("odd!")
What does this print?
Or reveal the answer without checking

Answer:odd!
is_even(4) prints True inside itself, but has no return statement — so it returns None to the caller. The outer if treats None as falsy, so the else branch runs and prints 'odd!', even though 4 is even.

Modification challenge: fix is_even so if is_even(4): correctly takes the if branch, by replacing the print calls inside it with return statements.

Summary

  • A function packages steps behind a name; its parameters are the inputs it expects, and return is the value it sends back.
  • Each call to a function gets its own fresh copies of its parameters — separate calls never interfere with each other.
  • Printing inside a function only shows something on screen; it does not send a value back to the caller. Without an explicit return, a function always returns None.
  • Tracing a call means following a value in through the parameters and back out through the return value.

Key terms