Port typeshed's tests/check_new_syntax.py here

[Akuli]

Typeshed currently has a script that checks a bunch of things that belong here IMO: https://github.com/python/typeshed/blob/af8e37d2736d3a6fbed8f5dfad71580dd417c612/tests/check_new_syntax.py

• Collections classes in typing: typing.Counter --> collections.Counter etc (related: Suggest using collections.abc etc, not typing (deprecated since 3.9) #46)
• Built-in generics: List --> list etc
• typing.(Async)ContextManager --> contextlib.Abstract(Async)ContextManager
• Preferring typing over typing_extensions when available: typing_extensions.ClassVar --> typing.ClassVar etc
• Union[str, int] --> str | int (Suggest using PEP 604-style union types #45)
• Banning from collections.abc import Set, because it's confusingly different from the built-in set
• Ordering sys.version_info checks so that the most recent version is first
• Ban explicitly inheriting from object in Python 3-only stubs.
• Disallow using typing.Text in Python 3-only stubs.

We should probably create one error code per Python version, so that it's easy to e.g. enable errors for not using features new in Python 3.8+. Another option would be to make the errors depend on sys.version_info (maybe too clever, would need good documentation).

[Akuli]

Another thing to consider, maybe as a separate issue: Much of these checks would make sense for .py files too. For example, I don't use Optional in my projects, unless I use a library that evaluates type annotations at runtime, and it would be awesome if the CI would check this.

[AlexWaygood]

Another thing to consider, maybe as a separate issue: Much of these checks would make sense for .py files too. For example, I don't use Optional in my projects, unless I use a library that evaluates type annotations at runtime, and it would be awesome if the CI would check this.

I'm a little wary of mission creep -- I kinda like that this is a very small, focused package. Plus, I think maybe https://github.com/asottile/pyupgrade already does a bunch of these things? :)

[Akuli]

Thanks for reminding me about pyupgrade. It indeed seems better for .py files, and it already has some typing things. Maybe we should focus on things that don't have anything to do with pyupgrade, e.g. banning from collections.abc import Set.

[AlexWaygood]

To be clear: I agree it would be nice to include all these things for .pyi files, even if pyupgrade already has some of these features, I'm just not necessarily convinced we should venture into checking .py files as well.

[AlexWaygood]

#97 has ported most of the import checks from check_new_syntax to flake8-pyi. However:

1. check_new_syntax bans importing collections.OrderedDict from typing or typing_extensions. That hasn't yet been implemented in flake8-pyi, because it can't be applied to stubs supporting Python 2 (OrderedDict can be imported from typing_extensions in Python 2, but not from collections).
2. check_new_syntax bans importing ContextManager from typing or typing_extensions. That also hasn't yet been implemented in flake8-pyi, for the same reason: contextlib.AbstractContextManager doesn't exist in Python 2.

check_new_syntax works around these issues by just not checking the Python 2 files in typeshed.

[Akuli]

We could configure file-specific ignores in typeshed's .flake8.

[AlexWaygood]

We could configure file-specific ignores in typeshed's .flake8.

I think that's the way to go, too.

I'd like to have them in a different error code, so that we can switch off the minimum possible number of checks for Python 2. Sound good?

[AlexWaygood]

The only one left to do here is:

Ordering sys.version_info checks so that the most recent version is first

For some reason I don't really feel like that "fits in" with the rest of flake8-pyi, so I'm probably not going to work on it myself. I'm happy to review code if somebody else wants to work on it, though.

[AlexWaygood]

Closing this as completed, since there doesn't appear to be any interest in implementing the final item on the list.