How to Make Mistakes in Python

Chapter 4. Structure

It’s a trap!

Admiral Ackbar

Let’s move on into questions of structure and how you can hurt your future self with tangled logic and deep coupling. These structural problems impact your ability to change or reuse your code as its requirements inevitably change.

Pathological If/Elif Blocks

This anti-pattern arises when you get into the business of creating a “one-stop shop” function that has to contend with many special cases.

The first if/else block arrives innocently, and even the first elif doesn’t seem so bad. But soon their friends arrive:

def do_awesome_stuff():
    ...
    if thing.has_condition_one():
        ...
    elif thing.has_condition_two():
        ...
    elif thing.get_conditions() in ['conditon3', 'condition4']:
        ...
    elif thing.has_condition_forty_two():
        ...
    else:
        ...
    ...

Suddenly you find yourself with hundreds of lines of elifs. And good luck if any of the contents of those blocks is at all complicated—anyone reading this code will be fortunate if they even remember they’re in this elif nightmare after 30 or 40 lines. And how excited will you be to write tests for this function?

This has a kind of momentum as well—special cases tend to attract more special cases, as if drawn together gravitationally. Just adding more elifs feels easier than cleaning up. Except cleaning up isn’t so bad. If we really do need to manage many special cases, we can employ the Strategy pattern:

def strategy1():
    ...

def strategy2():
    ...

strategies = {
    'condition1': strategy1,
    'condition2':