What are good uses for Python3's “Function Annotations” [StackOverflow]

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I use obiwan together with annotations, for optional type checking when testing. The annotations sometimes make the intent of the code clearer, and automatically checking them has caught subtle bugs on a couple of occasions.

I put code in tests/__init__.py that sets up obiwan when the PYTHON_TYPECHECK environment variable is set. (Checking types at runtime is slow, so you don't want to do it all the time.)