try from typing import ParamSpec except from typing_ext import ParamSpec not working

[ghost]

Bug Report
try-except imported ParamSpec is unusable.

try:
    from typing import ParamSpec
except ImportError:
    from typing_extensions import ParamSpec

This makes ParamSpec("P") become an invaild type.

To Reproduce

from typing import Callable

try:
    from typing import ParamSpec  # type: ignore[attr-defined,unused-ignore]
except ImportError:
    from typing_extensions import ParamSpec  # type: ignore[import,unused-ignore]

P = ParamSpec("P")

def func(arg: Callable[P, None]) -> None:
    return None

Expected Behavior
No error

Actual Behavior
error: The first argument to Callable must be a list of types, parameter specification, or "..." [valid-type]

Your Environment

• OS: Ubuntu 22.04.3 LTS (GNU/Linux 5.15.0-94-generic x86_64)
• Mypy version used: 1.8.0 (compiled: yes)
• Mypy command-line flags: None
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.8.18

Reproduce repository

[hamdanal]

This way of conditional import is not currently supported by mypy. You'll have to use a version check to do the import.
This code achieves the same thing and type checks correctly by mypy:

import sys
from typing import Callable

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

P = ParamSpec("P")

def func(arg: Callable[P, None]) -> None:
    return None

You can read more about it here
https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-new-additions-to-the-typing-module

[AlexWaygood]

Duplicate of #14220

[ghost]

Hmm... it doesn't works very well.
Once I use from sys import ...:

from sys import version_info

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

Mypy says 6:5: error: Incompatible import of "ParamSpec" (imported name has type "type[typing_extensions.ParamSpec]", local name has type "type[typing.ParamSpec]") [assignment]

[AlexWaygood]

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