Support file-level assertions as inline configuration

[ofek]

Feature

One would be able to do:

# mypy: assert=platform == "win32" and python_version >= "3.10"

and the file would only be checked if sys.platform is win32 and the tuple sys.version_info is greater than or equal to (3, 10). This is similar to the environment markers used in dependency definitions but with only support for the two static platform variables that are already used by Mypy.

Implementation-wise, there would be a pre-processing step that transforms the Python version string (if present) into a tuple. This way you can pass those two variables directly to eval.

Pitch

This would significantly help in situations such as #19009. Brief recap:

When you have the following provider module:

if sys.platform == "win32":
    from my_pkg._pty.windows import PtySession
else:
    from my_pkg._pty.unix import PtySession

and attempt importing this symbol from somewhere else, Mypy does not respect the platform condition.

For example, if you call pty.openpty() inside my_pkg._pty.unix and run Mypy on Windows it will show:

Module has no attribute "openpty"  [attr-defined]

The best solution available is adding a top-level assertion as introduced here. For example, on top of the module my_pkg._pty.unix you would put assert sys.platform != "win32".

This is suboptimal because it requires modifying runtime behavior. The sys import is required, perhaps only for the assertion, and if there are imports that only exist conditionally the assertion has to be in the middle of the standard import block on top.

Adding a file-level comment is the most pragmatic option at hand, in lieu of adding an option that automatically propagates conditional static assertions to enclosed imports.

[sterliakov]

It might be easier to support the following idiom:

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    import sys
    
    assert sys.platform == 'linux'

as equivalent to top-level assert. It won't be a trivial change, but might still be easier than assert.

(beware of https://peps.python.org/pep-0781/ - should it get accepted, typing import won't be necessary too, this will be almost a no-op at runtime - simple if condition evaluation, insignificant for the import time)

[ofek]

That would indeed be nicer than the current situation but it still requires 3 lines of code and would still have to be inserted before imports that don't exist without the condition.

[dlech]

if TYPE_CHECKING:

I was hoping this would work but it doesn't.

Compare https://mypy-play.net/?mypy=1.15&python=3.12&gist=b11740ebcbc73e8300ba64eb80eafd28

from typing import TYPE_CHECKING

if TYPE_CHECKING:
    import sys
    
    assert sys.platform == 'win32'
    
from ctypes import WinError

main.py:8: error: Module "ctypes" has no attribute "WinError"  [attr-defined]
Found 1 error in 1 file (checked 1 source file)

To https://mypy-play.net/?mypy=1.15&python=3.12&gist=ebd7a55f45d19c31df4fd56d7911c5f3

import sys

assert sys.platform == 'win32'
    
from ctypes import WinError

Success: no issues found in 1 source file

I can't use the latter because it breaks Sphinx autodoc by raising an AssertionError since it imports the module.

[dlech]

I found something that works a bit better.

if TYPE_CHECKING:
    if sys.platform != "win32":
        assert False, "This backend is only available on Windows"

But it doesn't suppress all errors, for example from ctypes import WinError after this still causes an error.