Functions and scope

A function is not mainly a way to avoid repeating yourself. It is a way to give a decision a name, so the rest of the program can stop thinking about it.

def is_overdue(due_day, today):
    return today > due_day

Now the rest of the program says if is_overdue(d, t): and nobody has to remember whether the comparison was > or >=. The decision is made in one place, and one place is where you go to change it.

Parameters and arguments

A parameter is the name in the definition. An argument is the value you pass in. They are different words because they are different things: the parameter is local to the function, and assigning to it inside does not touch whatever was passed in.

Scope, in one rule

A name created inside a function is visible only inside that function, and disappears when the function returns.

def tally(items):
    count = 0        # local to tally
    for item in items:
        count += 1
    return count

print(count)         # NameError: count is not defined

That error is not the language being awkward. It is the language keeping the promise that reading tally tells you everything tally does.

Returning versus printing

print shows a value to a person. return hands a value back to the program. A function that prints its answer and returns None cannot be used in a larger expression, and this is the single most common structural mistake in week 8.