add mapping & sequence type flags to the stable api

[davidhewitt]

In #25723 (comment) it was implied that these flags should be "internal", but it's not entirely clear to me why. PEP 634 doesn't make a clear indication whether they should be in the stable API or not.

These flags are already readily available when compiling C extensions with the full API and can be used in two ways:

• to implement native classes which can be used as mappings in match cases
• to test for sequence / mapping types in the same way that the match keyword would

It would be nice to have them available in the stable ABI, if that's deemed reasonable.

[davidhewitt]

Hi, is there someone available who would be willing to review this please? Maybe @markshannon as the original author of these flags?

[kumaraditya303]

Please create an issue to discuss this change. Thanks

[TeamSpen210]

That comment doesn't look like it's describing the flags itself, but instead whether they should be present as public constants in collections.abc itself, which it shouldn't. Type flags aren't part of the language, just the C API.

Instead of using the flags, an alternative for the equivalent functionality would be to do the same way Python code does - import collections.abc.Mapping/Sequence, then call .register() or PyObject_IsInstance()/PyObject_IsSubclass(). Less performant of course, but it'll work on pretty much any Python version.

[markshannon]

I don't think these flags should be part of the stable API.

As @TeamSpen210 says, using collections.abc.Sequence.register() is more robust, as it ensures consistency of flags and abstract base class.

[davidhewitt]

Understood, thanks for the feedback - will close this 👍