Mention Ellipsis pickling in the docs

[sterliakov]

I literally came to docs to check if Ellipsis can be pickled and unpickled, but had to try it in REPL instead. This micro PR updates the documentation page to mention that Ellipsis is picklable. (no issue code, since this is a minor doc-only change)

Needs backport to all maintained versions, because this behaviour did not change (at least it's picklable in versions 3.7 to 3.12, I haven't checked earlier versions).

[AA-Turner]

Is this covered by ‘classes accessible from the top level of a module’? For example, NotImplemented isn’t contained in this list. Perhaps the ‘special’ thing about Ellipsis is the relation to ...?

A

[sterliakov]

Oh, I'd rather add NotImplemented too, thanks for the reminder! Just checked, and it's picklable as well.

Neither Ellipsis nor NotImplemented are covered by "classes accessible from..." at least because they are not classes... These five builtin singletons (with None, True, and False) are special due to their nature. In fact ... and NotImplemented are covered by "instances of such classes whose the result of calling __getstate__() is picklable", but this finding requires treating them in slightly unusual way (Ellipsis is not "instance of type types.EllipsisType" from the developer point of view, but a builtin truthy literal singleton, and the same logic applies to NotImplemented - instance of types.NotImplementedType).

[AA-Turner]

Oh, I'd rather add NotImplemented too, thanks for the reminder!

Just added a review suggestion on that point.

Good points about classes vs singleton instances of the types!

A

[hauntsaninja]

If we're documenting this behaviour, ideally we should make sure it's tested as well.

[sterliakov]

Sorry for having abandoned this PR for a while. All these cases (both NotImplemented and Ellipsis) are already covered by tests introduced in BPO6477 9 years ago together with None pickling test.

cpython/Lib/test/pickletester.py

Lines 1976 to 1983 in d08d49d

	def test_singleton_types(self):
	# Issue #6477: Test that types of built-in singletons can be pickled.
	singletons = [None, ..., NotImplemented]
	for singleton in singletons:
	for proto in protocols:
	s = self.dumps(type(singleton), proto)
	u = self.loads(s)
	self.assertIs(type(singleton), u)

[AA-Turner]

@hauntsaninja I've merged HEAD as the branch was out-of-date, so the checks would never succeed (& hence auto-merge would never happen)

A

[miss-islington]

Thanks @sterliakov for the PR, and @hauntsaninja for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[bedevere-bot]

GH-108661 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

GH-108662 is a backport of this pull request to the 3.11 branch.