fixed NotImplementedType regression

[randolf-scholz]

Fixes #14965

[github-actions]

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

static-frame (https://github.com/static-frame/static-frame)
+ static_frame/core/node_values.py:98: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/node_values.py:272: error: Unused "type: ignore" comment  [unused-ignore]

[randolf-scholz]

@srittau This should do the trick.

[hauntsaninja]

Does this intentionally drop the __call__: None?

[randolf-scholz]

@hauntsaninja I believe this is/was just some relic for annotating that this class couldn't be instantiated before @type_check_only.

[hauntsaninja]

iirc it was a trick that gave better error messages when users did raise notimplemented() instead of raise notimplementederror()

maybe worth re-running pyright and mypy to see what the errors are like these days

[randolf-scholz]

Oh, is that a thing? I've never even considered raising NotImplemented rather than NotImplementedError, it doesn't seem to work at runtime:

>>> raise NotImplemented()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'NotImplementedType' object is not callable
>>> raise NotImplemented
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: exceptions must derive from BaseException

[randolf-scholz]

Ok, so I just tested the following:

def testA() -> None:
    raise NotImplemented()

def testB() -> None:
    raise NotImplemented

with results (this PR)

	mypy 3.9	mypy 3.10	pyright 3.9	pyright 3.10
raise NotImplemented	ERROR	OK	OK	OK
raise NotImplemented()	ERROR	OK	ERROR	OK

vs commit 8aaa86f (before this PR)

	mypy 3.9	mypy 3.10	pyright 3.9	pyright 3.10
raise NotImplemented	ERROR	ERROR	OK	OK
raise NotImplemented()	ERROR	ERROR	ERROR	OK

• the __call__: None means that raise NotImplemented() will generate an error of the form error: "None" not callable [misc]
  • How useful is this? Since we subclass Any, NotImplemented.method() or NotImplemented[key] will not raise any error either
  • raise NotImplemented already gets flagged by linters (flake8 rule F901, ruff F901)
• mypy no longer emits error: Exception must be derived from BaseException; did you mean "NotImplementedError" for 3.10 after this PR for 3.10+.
  • not sure why this changed...

[srittau]

When re-adding this, we should leave a comment explaining why it's there.

[hauntsaninja]

Okay, looks like mypy in the intervening years added some special casing here. This explains your last bullet point; the special casing was added assuming a fully resolved name of builtins._NotImplementedType.

Given mypy has some special casing now and pyright's behaviour is unaffected, let's just do the simpler thing and remove this in both places. I opened #14971 to make the definitions consistent in typeshed. Also opening a PR to mypy to fix up the special casing

[hauntsaninja]

mypy PR here python/mypy#20168