gh-104874: Document NewType.__supertype__

[JelleZijlstra]

Fixes #104874

• Issue: Document NewType.__supertype__ #104874

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📚 Documentation preview 📚: https://cpython-previews--104875.org.readthedocs.build/

[AlexWaygood]

Looks good. We also test for __qualname__ and __module__; should we document those as well?

cpython/Lib/test/test_typing.py

Lines 6658 to 6669 in c90a862

	def test_special_attrs(self):
	self.assertEqual(UserId.__name__, 'UserId')
	self.assertEqual(UserId.__qualname__, 'UserId')
	self.assertEqual(UserId.__module__, __name__)
	self.assertEqual(UserId.__supertype__, int)
	UserName = self.UserName
	self.assertEqual(UserName.__name__, 'UserName')
	self.assertEqual(UserName.__qualname__,
	self.__class__.__qualname__ + '.UserName')
	self.assertEqual(UserName.__module__, __name__)
	self.assertEqual(UserName.__supertype__, str)

[JelleZijlstra]

__qualname__ is actually wrong as implemented (it's always the same as __name__) and __module__ is mostly an implementation detail for pickling, so I'd prefer to leave those undocumented.

[AlexWaygood]

__module__ is mostly an implementation detail for pickling

I feel like it could be useful to be able to know the module the newtype was defined in, and it seems useful to mention the ways in which instances of NewType are similar to classes, even though they're not classes.

[miss-islington]

Thanks @JelleZijlstra for the PR, and @AlexWaygood for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[bedevere-bot]

GH-104906 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

GH-104907 is a backport of this pull request to the 3.11 branch.