When writing functions, it's very useful to follow guidelines so that you write them well. I'll quickly point some of them out:

• Functions should do one thing: Functions that do one thing are easy to describe in one short sentence. Functions that do multiple things can be split into smaller functions that do one thing. These smaller functions are usually easier to read and understand. Remember the data science example we saw a few pages ago.
• Functions should be small: The smaller they are, the easier it is to test them and to write them so that they do one thing.
• The fewer input parameters, the better: Functions that take a lot of arguments quickly become harder to manage (among other issues).
• Functions should be consistent in their return values: Returning False or None is not the same thing, even if within a Boolean context they both evaluate to False. False means that we have information (False), while None means that there is no information. Try writing functions that return in a consistent way, no matter what happens in their body.
• Functions shouldn't have side effects: In other words, functions should not affect the values you call them with. This is probably the hardest statement to understand at this point, so I'll give you an example using lists. In the following code, note how numbers is not sorted by the sorted function, which actually returns a sorted copy of numbers. Conversely, the list.sort() method is acting on the numbers object itself, and that is fine because it is a method (a function that belongs to an object and therefore has the rights to modify it):

>>> numbers = [4, 1, 7, 5]
>>> sorted(numbers)  # won't sort the original `numbers` list
[1, 4, 5, 7]
>>> numbers  # let's verify
[4, 1, 7, 5]  # good, untouched
>>> numbers.sort()  # this will act on the list
>>> numbers
[1, 4, 5, 7]

Follow these guidelines and you'll write better functions, which will serve you well.