rewrite NoneType to None

[carljm]

No description provided.

[m1schka]

not sure if it's related but I also have applied the types and it generated NoneType as a parameter, which is unknown to standard python and not getting added to the import (the only option would be to import it from monkeytype but I actually don't want monkeytype imports in my business code).

[iamrajhans]

@carljm can i take this one

[carljm]

@m1schka Yeah, that's this bug. We have

MonkeyType/monkeytype/stubs.py

Line 327 in d36c344

elif anno is NoneType:

but it seems it's not effective (at least not in all cases). The correct fix here is that we should never generate NoneType at all in an annotation, it should always be translated to None.

@iamrajhans Sure, thank you!

[johnarnold]

I wrote a test case for this. It happens when types like Optional[str, NoneType] or even more complex occur.
My example:

    def test_union_nonetype_annotation(self):
        """NoneType should always be rendered as None"""
        sig = Signature.from_callable(UpdateSignatureHelper.has_annos)
        sig = update_signature_args(sig, {'a': Dict[str, Dict[str, Union[str, NoneType]]]}, has_self=False,
                                    ignore_existing_annotations=True)
        stub = FunctionStub('test', sig, FunctionKind.MODULE)
        expected = 'def test(a: Dict[str, Dict[str, Union[str, None]]], b) -> int: ...'
        assert stub.render() == expected

All the magic happens in render_annotations():
https://github.com/Instagram/MonkeyType/blob/master/monkeytype/stubs.py#L333

I'll submit a PR since this keeps biting me.