Functions, parameters and scope

Defining and calling a function, passing it arguments, getting its return value, and knowing where a variable exists: local, global, shadowed.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Function: what is it?
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A named block of statements, called as many times as needed with different data, which returns a result.
You define it once with def, then call it by its name followed by parentheses: area(3).
What does this program print?

```python
def double(x):
print(x * 2)

r = double(4)
print(r)
`
- 8 then None
- 8 then 8
- 8 only
- None only
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8 then None
The call prints 8, but the function has no return: it returns None, which r receives. Printing a value is not returning it.
A … is a subprogram that acts (printing, saving) without returning a result; in Python, calling it gives ….
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A procedure is a subprogram that acts (printing, saving) without returning a result; in Python, calling it gives None.
Python's syntax does not tell the two apart: a procedure is a function with no return, or whose return gives nothing back.
Formal parameter: what is it?
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A name written in a function's header, which receives a value at each call: radius in def area(radius):.
It is a local variable of the function, initialised by the call.