mypy --warn-unused-ignores doesn't work

[srittau]

I'm unsure about the reason, but when I locally run mypy --warn-unused-ignores stdlib/builtins.pyi (or any other file in stdlib or stubs) where I have manually added an unnecessary # type: ignore, I get no warnings. mypy is current in my venv (0.910) and I deleted .mypy_cache to make sure. The same happened when I ran ./tests/mypy_test.py --warn-unused-ignores, which is how I noticed.

Does anyone else have this problem?

[srittau]

And I just double checked: If I create a file foo.pyi in my home directory:

def foo() -> None: ...  # type: ignore

and run mypy ~/foo.pyi --warn-unused-ignores from the typeshed directory, I get a warning. As soon as I move the file to the top-level typeshed directory, I don't get a warning.

[srittau]

And another data point: When I run mypy foo.pyi --warn-unused-ignores in the typeshed dir, then move the file to my home directory, then run mypy ~/foo.pyi --warn-unused-ignores, I don't a warning unless I delete .mypy_cache.

[Akuli]

Happens to me too.

(env) akuli@akuli-desktop:~/typeshed$ mypy --warn-unused-ignores stdlib/builtins.pyi 
Success: no issues found in 1 source file
(env) akuli@akuli-desktop:~/typeshed$ rm -rf .mypy_cache/
(env) akuli@akuli-desktop:~/typeshed$ mypy --warn-unused-ignores stdlib/builtins.pyi 
Success: no issues found in 1 source file
(env) akuli@akuli-desktop:~/typeshed$ git diff
diff --git a/stdlib/builtins.pyi b/stdlib/builtins.pyi
index 6bc58d2a..52fa409d 100644
--- a/stdlib/builtins.pyi
+++ b/stdlib/builtins.pyi
@@ -269,10 +269,10 @@ class float:
     def is_integer(self) -> bool: ...
     @classmethod
     def fromhex(cls, __s: str) -> float: ...
-    @property
-    def real(self) -> float: ...
-    @property
-    def imag(self) -> float: ...
+    @property  # type: ignore
+    def real(self) -> float: ...  # type: ignore
+    @property  # type: ignore  # type: ignore
+    def imag(self) -> float: ...  # type: ignore
     def conjugate(self) -> float: ...
     def __add__(self, __x: float) -> float: ...
     def __sub__(self, __x: float) -> float: ...

[AlexWaygood]

Same for me, on Windows.

[hauntsaninja]

You'll want to comment out https://github.com/python/mypy/blob/50db29a527595822fe6f76053a6a54f5c301649a/mypy/errors.py#L465
Note that you'll run into python/mypy#8823, so would need to do some kind of intersection.

Okay, intersecting 3.6 and 3.10 I get:

{'stdlib/_msi.pyi:13',
 'stdlib/_msi.pyi:14',
 'stdlib/_msi.pyi:22',
 'stdlib/_msi.pyi:23',
 'stdlib/_msi.pyi:31',
 'stdlib/_msi.pyi:32',
 'stdlib/_msi.pyi:43',
 'stdlib/_msi.pyi:44',
 'stdlib/_operator.pyi:154',
 'stdlib/builtins.pyi:92',
 'stdlib/builtins.pyi:975',
 'stdlib/importlib/metadata.pyi:77',
 'stdlib/typing.pyi:693',
 'stdlib/typing_extensions.pyi:71',
 'stdlib/weakref.pyi:54'}

From a skim, I think something or the other would complain if you removed them, but please feel free to try,

[JelleZijlstra]

We figured out the reason for mypy's behavior, so this can be closed.

[yabberyabber]

@JelleZijlstra Could you provide a little more detail?

Are we using mypy incorrectly? Or is this a bug in mypy that may be fixed in a future release? Is there another issue or PR that might provide more detail?

[JelleZijlstra]

mypy has specific logic that suppresses this error in typeshed.

[hauntsaninja]

mypy intentionally doesn't respect --warn-unused-ignores in typeshed as linked^^: https://github.com/python/mypy/blob/50db29a527595822fe6f76053a6a54f5c301649a/mypy/errors.py#L465

Enabling --warn-unused-ignores would be problematic, because some type ignores are for other type checkers that use typeshed, or because we have conditional logic on sys.version. The list of potentially removable ignores is in my previous comment, none of them looked actually removable to me.