Python - Mypy
My new job has me working on a larger codebase than previous jobs, and it’s also my first time using mypy.
I’m starting to understand why Guido’s work on mypy had a lot to do with Dropbox’s Python 3 migration. Dropbox wrote an article with details on how they used mypy on their “4 million lines of Python”. With that much code, I can understand why they needed to treat code like “cattle, not pets”.
Python 3 disallows some comparisons with
None types, makes huge changes to strings/bytes, and has many other changes involving types. When your codebase is massive, you have to reach for automated tooling to consistently find and prevent those bugs. With the same tooling you can also prevent entire categories of other bugs from reaching production. The type hints can also be helpful documentation. For those reasons and more, I think implementing type hints on a large codebase like Dropbox’s will definitely be worth it in the long run.
It has me wondering if I should have been taking the extra time to use type hints and checking (or a statically typed language) this whole time. Was going without type hints one less distraction? Or will be the price be paid in maintenance difficulties and bug fixes later?
At this point, not much of the Python ecosystem has type hints (not even Python 2 compatible comment-style type hints). I’m starting to think it would be a good use of time to work on changing that.