Add implicit -> None type annotation to __init__ methods

[ArneBachmann]

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

[eindiran]

What args are you passing to mypy? I am asking because I just compiled an example without an annotation and ran mypy with my standard set of mypy args, and I did not see a warning.

My standard mypy args: '--ignore-missing-imports', '--python-version', '3.5', '--warn-incomplete-stub', '--warn-redundant-casts', '--warn-return-any', '--warn-unused-ignores'

[evhub]

@ArneBachmann I can't reproduce this; can you give me some example code that replicates this error?

[ArneBachmann]

I don't use any of those mypy options, but it seems to have gone away, was probably earlier a thing with Python 2 code compat problems, that made me make it a habit to mark every procedure with -> None. I recently decided to ditch all Python 2 support as it was really hard to achieve, even with Coconut. Just wasn't worth it anymore, especially since any reliable Python console app on Windows requires PEP528/529, supported only from Python 3.6 on (!).

[ArneBachmann]

I have it again:

sos/utility.py:65: error: The return type of "__init__" must be None

for

def __init__(_, mapping:Dict[str,Any]): dict.__init__(_, mapping)

Any idea?

[auscompgeek]

__init__ should always be annotated with a return type of None, as per PEP 484. Not a bug.

[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.

[evhub]

One possible solution to this is to have Coconut automatically annotate __init__ definitions with a return type of None. Thoughts?

[ArneBachmann]

+1 since also class definitions may leave out the : pass part, similar how -> None could be left out.

[baodrate]

python/mypy#5677

[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.