Defining Functions

Package code into reusable functions with inputs and outputs.

Loading your progress…Next: 4.2 Arguments and Scope

In this lesson9 parts

01The same lines, twice
02def, and calling a function
03What a call does
04return, print and None
05return ends the function
06Returning several values
07Docstrings and type hints
08Built-in and user-defined, and the first mistakes
09Recap, your challenge, and next

Key terms

The words this lesson introduces, each in one line. The module’s glossary collects them all.

argument: A value passed to a function in a call: 12.50 in with_tax(12.50).
call: Running a function's body, by writing its name and round brackets: with_tax(12.50).
docstring: A string on the first line of a function's body that says what the function does. help() and __doc__ show it.
frame: The namespace one call runs in. It holds the parameters and the names the body binds, and is removed when the call returns.
function definition: A def statement; running it creates a function object and binds its name. The body does not run until a call.
parameter: The name in a function's definition that a value will be bound to: price in def with_tax(price):.
return value: The value a call gives back. A function with no return gives back None.
type hint: A note in a function's header of the type a parameter should have, or the type it returns: review: str, -> list[str]. Python does not check it when the code runs.

Quiz 5 questions

Your first pick on each question is the one that counts, and a right one earns a coin. Getting one wrong here is how the lesson sticks.

Practice

Problems to solve in your own notebook. Each states the problem, not the steps: working out the steps is the exercise. Level A applies the lesson, B combines it with earlier ones, C stretches it.

The self-checking notebook for this lesson is Notebook 1 · Functions and Arguments.

Common mistakes

What you will see when it goes wrong, why it happens, and the fix.

Where it’s used

Where this lesson’s ideas turn up in real work.