This is to implement the deprecation of using ABCs from collections:

Deprecated since version 3.3, will be removed in version 3.10: Moved Collections Abstract Base Classes to the collections.abc module. For backwards compatibility, they continue to be visible in this module through Python 3.9.

python/cpython@c47c78b#diff-c92f871cb9b4aa1583c89f82cd416d58a5d70fff41faf41e45dedf775dbe7713

So what is the correct fix? If you don't import these types, there are a bunch of undefined symbols referenced in this file.

The CI tests iterate through different versions of Python, but I suspect that they are not covering Python 3.10 because this would have never passed the CI tests.

The correct fix should be to import what is needed without re-exporting. Yes, that's correct — we currently don't run Python 3.10 in CI.

While I think this latest attempt will pass the current CI tests, I suspect it will break as soon as you start running the mypy tests with Python 3.10.

Yup. I already opened a PR to fix failures in the stdlib, your test failure conveniently shows what changes need to be made in third party stubs.

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

poetry (https://github.com/python-poetry/poetry.git)
+ poetry/utils/helpers.py:26: error: Module 'collections' has no attribute 'Mapping'
+ poetry/utils/helpers.py:26: error: Name 'Mapping' already defined (possibly by an import)

parso (https://github.com/davidhalter/parso.git)
+ parso/python/tree.py:49: error: Module 'collections' has no attribute 'Mapping'
+ parso/python/tree.py:49: error: Name 'Mapping' already defined (possibly by an import)

dulwich (https://github.com/dulwich/dulwich.git)
+ dulwich/config.py:44: error: Module 'collections' has no attribute 'Iterable'
+ dulwich/config.py:44: error: Module 'collections' has no attribute 'MutableMapping'
+ dulwich/config.py:44: error: Name 'Iterable' already defined (possibly by an import)
+ dulwich/config.py:44: error: Name 'MutableMapping' already defined (possibly by an import)


python-htmlgen (https://github.com/srittau/python-htmlgen.git)
+ htmlgen/element.pyi:51: error: Class cannot subclass 'Sized' (has type 'Any')
+ htmlgen/element.pyi:51: error: Name 'collections.Sized' is not defined
+ test_htmlgen/attribute.py:279: error: unused 'type: ignore' comment

@hauntsaninja, I've updated this PR to (I think) fix the incompatibilities that mypy_primer identified. Is there a way to trigger a new run to verify that no more issues are found?

Thanks for this!