Mypy: Cannot silence "unused 'type: ignore'" in version specific code

[serhiy-storchaka]

I need to execute some code only on specific Python version (it is the use of TextIOWrapper.reconfigure() added in Python 3.7, but in following example I will use more abstract code).

import sys

if sys.version_info >= (3, 7):
    x: int = 'a'

Since MyPy on Python 3.7 raises an error for it (actually because of the error in typeshed) I need to add # type: ignore:

import sys

if sys.version_info >= (3, 7):
    x: int = 'a'  # type: ignore

But now I will get an error unused 'type: ignore' comment when run MyPy with warn_unused_ignores = True on Python 3.6.

So I cannot use variant 1, because it is an error on 3.7, and cannot use variant 2, because it is an error on 3.6.

MyPy should not complain about unused 'type: ignore' comment in the code which it does not analyze.

MyPy version is 0.770.

[hauntsaninja]

As a workaround for this issue, you can use cast

[serhiy-storchaka]

It does not help in my particular issue.

The problem is that MyPy skips some parts of the code and complains about unused type: ignore in this code (which would be not ignored if this code be not skipped). It is not consistent and does not have a workaround.

[hauntsaninja]

Agree that there is an issue, but I think it can be worked around using cast, since that lets you remove the type ignore altogether:

~/delete λ cat test84.py
import sys
from typing import cast, Any

if sys.version_info >= (3, 7):
    x: int = cast(Any, 'a')
~/delete λ mypy --python-version 3.6 --warn-unused-ignores --warn-redundant-casts test84.py
Success: no issues found in 1 source file
~/delete λ mypy --python-version 3.7 --warn-unused-ignores --warn-redundant-casts test84.py
Success: no issues found in 1 source file

Looking at the issue on typeshed, you'd need to do something like cast(Any, sys.stdout).reconfigure(errors="replace"). It's not pretty, but it should work.

[serhiy-storchaka]

x: int = 'a' was just an example. It is the simplest way that I know to trigger an error. In real code I use

sys.stdout.reconfigure(errors="replace")

and I do not know how to silence an error in this case.

[serhiy-storchaka]

Oh, and thank you for your suggestion. I tried similar workaround, but made an error in the code, so it did not work. Now I see my mistake and fixed it thank to you.

[msullivan]

I think we need a way to silence unused type ignore messages for individual type ignores short of disabling the warning.

[wsanchez]

I'm running into a by-platform equivalent of this. If I try to import epoll from the select module on macOS, I get an [attr-defined] error, but on Linux, that's fine. Conversely, on Linux, one can't import KQ_FILTER_READ from select.

So, depending on which platform you run mypy, you get a different error, and I don't know how to suppress that class of error message.

That might be a different ticket, but it seems similar enough that maybe mentioning it in this one will do…?

[wsanchez]

Note the select issue I mention above appears to be new between Mypy 0.770 and 0.782.

[gvanrossum]

@wsanchez That is different. Are you using the right syntax for the platform check? Please ask on Gitter.

[wsanchez]

@gvanrossum Oh, OK, I didn't realize that Mypy could be told to do platform-specific things. I'll read more. https://github.com/twisted/twisted/pull/1416#issuecomment-702425277 describes the issue in a bit more detail.