More helpful type guards

[A5rocks]

A TODO I found while looking at something else got me down this rabbit hole, and I noticed mypy's subpar performance in these two cases.

Short descriptions from before I did this, so may be technically inaccurate:

1. check when defining a type guard whether the function is right or not
2. follow kw args if they go to a regular positional first arg

This may fix an issue but I have not conducted any searches.
Fixes #13199
Refs #14425

[A5rocks]

mypy primer results seem to be narrowing on self:

    @abstractmethod
    def is_error(self) -> TypeGuard[Error[_TSource, _TError]]:
        """Returns `True` if the result is an `Error` value."""

        raise NotImplementedError

This seems... wrong? Firstly, the PEP says this (as mentioned in one of my commit messages):

If a type guard function is implemented as an instance method or class method, the first positional argument maps to the second parameter (after “self” or “cls”).

This was even explicitly rejected! https://peps.python.org/pep-0647/#narrowing-of-implicit-self-and-cls-parameters

Secondly, mypy doesn't even support this!! At the moment:

from typing import TypeGuard

class A:
  def n(self) -> TypeGuard[int]:
    return False

a = A()
if a.n():
  reveal_type(a)  # N: Revealed type is "__main__.A"

---

However, I realize I missed staticmethod special casing. Will fix.

from typing_extensions import TypeGuard

class Z:
    @staticmethod
    def typeguard(h: object) -> TypeGuard[int]:  # E: TypeGuard functions must have a positional argument
        return isinstance(h, int)

x: object
if Z().typeguard(x):
    reveal_type(x)
if Z.typeguard(x):
    reveal_type(x)

[sobolevn]

Thanks! I have a couple of questions, this is not a full review.

[A5rocks]

OK with that tests should be added as requested. I've got one additional question that I've moved over to #14273 because it may cause more debate. Hopefully this as it stands is alright!

[sobolevn]

Cases that error:

   
    @abstractmethod
    def is_error(self) -> TypeGuard[Error[_TSource, _TError]]:
        """Returns `True` if the result is an `Error` value."""

        raise NotImplementedError

    @abstractmethod
    def is_ok(self) -> TypeGuard[Ok[_TSource, _TError]]:
        """Returns `True` if the result is an `Ok` value."""

        raise NotImplementedError

[A5rocks]

Yep! As mentioned before, that is invalid as according to the PEP and mypy doesn't support it anyways. Having an error at definition time is likely beneficial.

[A5rocks]

Someone ran into this in the issue tracker; I have updated the "fixes" section.

[hauntsaninja]

This looks great, thanks!

[hauntsaninja]

Not sure this return type makes sense. But can add an error in another PR

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

Tanjun (https://github.com/FasterSpeeding/Tanjun)
- version: 1.0.0+dev.7c14ebae6ca5d6ec39600e122b58a62afaa3ab02
+ version: 1.0.0+dev.d490f40dcc4f6244b2c486e9f6b6a7234763310f
-   File "/semanal.py", line 6235, in accept
+   File "/semanal.py", line 6249, in accept
-   File "/semanal.py", line 5909, in defer
+   File "/semanal.py", line 5923, in defer

Expression (https://github.com/cognitedata/Expression)
+ expression/core/result.py:125: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/result.py:131: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/result.py:212: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/result.py:217: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/result.py:323: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/result.py:327: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/option.py:139: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/option.py:144: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/option.py:214: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/option.py:218: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/option.py:330: error: TypeGuard functions must have a positional argument  [valid-type]
+ expression/core/option.py:334: error: TypeGuard functions must have a positional argument  [valid-type]

[hauntsaninja]

Oh hmm, the crash on FasterSpeeding/Tanjun is new (not to this PR, just new in general)