Review

Key Ideas

def defines a function.
A function does not run until it is called.
Arguments are values passed into a call.
Parameters are local names that receive arguments.
return sends a value back to the caller.
print() shows a value but does not return it.
Names created inside a function are local.
Defaults supply values when arguments are omitted.
Small tests use known inputs and expected outputs.
Docstrings help readers understand the function's contract.

Key Syntax

Syntax	Meaning
`def name():`	define a function
`name()`	call a function
`def f(x):`	define one parameter
`return value`	send a value back
`factor=1`	default parameter value
`f(value=8)`	keyword argument
`assert f(2) == 4`	small executable check
`"""Return ..."""`	docstring

Common Mistakes

Mistake	Fix
Defining a function but never calling it	add a call
Printing when later code needs the value	use `return`
Calling a function before defining it	put the definition first
Depending on hidden outer names	pass values as parameters
Expecting local assignments to change outer names	return the new value
Using a broad docstring to hide unclear names	improve the names

Checklist

You are ready to move on if you can:

define and call a function
explain argument versus parameter
predict a returned value
tell print() apart from return
trace a local name
use a simple default argument
write two small assert checks
write a one-sentence docstring

If You Feel Lost

Start with this:

Then change the input and predict the new output before running it.