Support conditionally importing from typing_extensions

[JukkaL]

This code tries to import TypeAlias from typing if possible and falls back to typing_extensions if it isn't available (e.g. on Python 3.8):

try:
    from typing import TypeAlias  # type: ignore
except ImportError:
    from typing_extensions import TypeAlias

Mypy doesn't actually support this as expected in older Python versions that are missing the definition in typing. They will just get an Any value, and the except block has no significance.

Perhaps the import from typing_extensions should take precedence? Or can we at least generate a more specific error about this?

A possible workaround is to flip the order of the imports.

This probably affects any feature that requires an import of a backported definition from typing_extensions.

Related issue: #14219

[erictraut]

Is there a need to support this idiom? The typing_extensions library already provides a backward-compatibility abstraction and uses typing.TypeAlias if it's available, so the above code could be simplified to from typing_extensions import TypeAlias. This is not only simpler to read and maintain, but it also eliminates problems for static analysis tools.

[hauntsaninja]

Spelling things with if sys.version_info checks works well; I'm fine with a won't fix here

[AlexWaygood]

the above code could be simplified to from typing_extensions import TypeAlias.

Some packages might prefer to only require the extra dependency if a user is installing the package on an older version of Python, so as to keep third-party dependencies to an absolute minimum where possible. I think this practice is relatively common.

Having said that, I agree with @hauntsaninja that the sys.version_info way of doing things is ~fine (though I do personally prefer EAFP style here).

[AlexWaygood]

@gamecss, please don't post duplicate comments in multiple places. Copying and pasting the reply I left you in #16903:

Correct. You need to do this:

import sys

if sys.version_info >= (3, 10):
    from typing import ParamSpec
else:
    from typing_extensions import ParamSpec

You can't do from sys import version_info and expect mypy to understand it in the same way, I'm afraid. It doesn't have the ability to understand arbitrary constructs like this; only a small selection of special-cased constructs are supported. See the relevant section in PEP 484: https://peps.python.org/pep-0484/#version-and-platform-checking