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]

@srittau This should do the trick.

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

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

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

Ok, so I just tested the following:

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

def testB() -> None:
    raise NotImplemented

with results (this PR)

vs commit 8aaa86f (before this PR)

• 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...

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

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

mypy PR here python/mypy#20168