warn_unused_ignores is too sensitive: false positives

[mitar]

If two packages both ignore an import, then one gets a false positive "unused ignore" error if warn_unused_ignores is enabled.

Reproduction: https://github.com/mitar/mypy-unused-ignores-issue

$ mypy bar/

$ mypy foo/
bar/__init__.py:1: note: unused 'type: ignore' comment

There is no error in bar package, but if I call mypy on foo, then an error in bar appears, but it should not. (Just because foo imports bar, and both import same 3rd party package which is ignored, once in foo, once in bar.)

[elliott-beach]

I think the repro could be simplified to anytime a bad package is imported and ignored multiple times, for example:

import numpy  # type: ignore
import numpy  # type: ignore

will trigger the unused ignore warning.
Is this really too sensitive, though? The behavior appears to be that once an import is ignored once, it is ignored globally, so multiple ignores are redundant, e.g.

import numpy  # type: ignore
import numpy

passes mypy.

If it would be more desirable to require imports to be ignored every time, then the behavior to change is how mypy checks imports, not how it checks redundant ignores.

[mitar]

I think the repro could be simplified to anytime a bad package is imported and ignored multiple times

I am not sure if such simplification is equivalent. So the issue here is that I want both bar package and foo to be able to be type-checked. If I run mypy bar, I need type: ignore to get it to type check. But if I run mypy foo, then this is an error. So there is not really a code change I could do to make this work in both cases. It is not the same as a redundant import. It is import in two different files.

Moreover, tricky thing here is that it depends on the order of how imports are encountered in the code. This is very unpredictable and so it is also hard to assure if you have multiple imports in various parts of the code (and even various packages).

[elliott-beach]

That's true, especially with respect to import orderings. Without the type: ignore in bar, I would expect mypy foo to warn for "no library stub in module numpy" from the import in bar, which it doesn't. So one way to resolve this would be to require imports to be ignored in each file they occur.

The simplest solution would be to specifically check import statements in generate_unused_ignores for a bad import that is ignored multiple times. Would a PR doing that be considered?

mypy/mypy/errors.py

Lines 308 to 316 in cbbde09

	def generate_unused_ignore_notes(self) -> None:
	for file, ignored_lines in self.ignored_lines.items():
	if not self.is_typeshed_file(file):
	for line in ignored_lines - self.used_ignored_lines[file]:
	# Don't use report since add_error_info will ignore the error!
	info = ErrorInfo(self.import_context(), file, self.current_module(), None,
	None, line, -1, 'note', "unused 'type: ignore' comment",
	False, False)
	self.error_info.append(info)

[posita]

Related to #5940 or #8823? (Or maybe those are special cases of this one?)