> Two very common initial problems I've seen are ... mypy is not running as part of the build ... If MyPy isn't running in your build/CI, it's possibly worse t

> Two very common initial problems I've seen are ... mypy is not running as part of the build ... If MyPy isn't running in your build/CI, it's possibly worse than useless. Until recently typed-annotated Python code wasn't checked in the build. The only time you'd be able to notice a problem was when the IDE's MyPy plugin showed a red squiggly. One I got MyPy integrated with Bazel we could run it over our codebase, and lo and behold there were at least a dozen errors which were actively misleading developers about the code they were reading. If something (MyPy, Pyre) isn't checking the type-annotations all the time, they're going to decay into something worse than untyped Python.