How to Make Mistakes in Python

Chapter 2. Silly Things

Oops! I did it again.

Britney Spears

There’s a whole category of just plain silly mistakes, unrelated to poor choices or good intentions gone wrong, the kind of strangely simple things that I do over and over again, usually without even being aware of it. These are the mistakes that burn time, that have me chasing problems up and down my code before I realize my trivial yet exasperating folly, the sorts of things that I wish I’d thought to check for an hour ago. In this chapter, we’ll look at the three silly errors that I commit most frequently.

Forgetting to Return a Value

I’m fairly certain that a majority of my hours spent debugging mysterious problems were due to this one simple mistake: forgetting to return a value from a function. Without an explicit return, Python generously supplies a result of None. This is fine, and beautiful, and Pythonic, but it’s also one of my chief sources of professional embarrassment. This usually happens when I’m moving too fast (and probably being lazy about writing tests)—I focus so much on getting to the answer that returning it somehow slips my mind.

I’m primarily a web guy, and when I make this mistake, it’s usually deep down in the stack, in the dark alleyways of the layer of code that shovels data into and out of the database. It’s easy to get distracted by crafting just the right join, making sure to use the best indexes, getting the database query just so, because that’s the fun part.

Here’s an example fresh ...