Add implicit -> None type annotation to __init__ methods

Should work fine with mypy even if not marked via -> None: (because it’s implicit in Python terms)

[eindiran]

That’s not entirely correct. PEP 484 specifies that __init__ should be annotated with a return type of None if annotated, but that “Python will remain a dynamically typed language, and the authors have no desire to ever make type hints mandatory, even by convention”.

As you can see in this mypy issue, Guido himself complains that “… it’d suck if we’d have to develop the habit of adding explicit ‘-> None’ to all init methods just to make mypy happy”. But the issue is still an open bug with mypy, so for the time being, making mypy happy requires that you explicitly add the ‘-> None’. Hopefully in the near future, __init__ will be assumed to have a default return type of None instead of Any, as mentioned in the final comment.

[auscompgeek]

Per the above, this is probably not worth implementing. If someone wants a type-checked no-argument __init__, then they should probably type hint the function signature.

If someone didn’t want their no-argument __init__ type-checked, then that should still be possible whilst still running mypy on their code.