Why you should write your tests first

We’re all agreed that any Python code that’s even a little serious needs unit tests, right? However, sometimes we end up writing our tests after we’ve written our code rather than doing test-driven development, what Curtis calls “code and cover”. That’s bad. Here’s why.

1. It’s dull. Really dull.
2. You find bugs, but it’s somehow more frustrating. Perhaps because you thought your code was correct already.
3. The code is probably not written for testability, which means you have to mix refactoring up with verifying behaviour. Messy & perilous.
4. Alternatively, you write tests with a lot of mocks. Not bad in itself, but risky.
5. It’s much harder to get full coverage.
6. You write tests for things that you don’t care about, just to exercise a particular code path. This makes the tests more fragile.
7. You never really know when you are finished.