`type: ignore` comments on imports of external package lead to `[unused-ignore]` error codes every other run

[provinzkraut]

Bug Report

When running mypy with --warn-unused-ignore --no-namespace-packages --warn-unused-ignores --ignore-missing-imports, putting a type: ignore[attr-defined] on an import will have mypy report an Unused "type: ignore" comment [unused-ignore] every other run, unless the cache is being cleared between runs.

To Reproduce

from opentelemetry.instrumentation.asgi import (  # type: ignore[attr-defined]
    OpenTelemetryMiddleware,
)

(Since this only seems to work with external packages, some setup is required)

1. Run pip install opentelemetry-instrumentation-asgi opentelemetry-distro
2. Add the code snippet below to file.py
3. Run mypy --warn-unused-ignore --no-namespace-packages --warn-unused-ignores --ignore-missing-imports file.py: Observer no error
4. Run mypy --warn-unused-ignore --no-namespace-packages --warn-unused-ignores --ignore-missing-imports file.py: Observe an [unused-ignore] error
5. Run mypy --warn-unused-ignore --no-namespace-packages --warn-unused-ignores --ignore-missing-imports file.py: Observer no error
6. Repeat the cycle as many times as you like :)

When removing the cache between runs, all works well, so I suspect it has something to do with caching.

Your Environment

• Mypy version used: 1.18.2, main
• Mypy command-line flags: --warn-unused-ignore --no-namespace-packages --warn-unused-ignores --ignore-missing-imports
• Mypy configuration options from mypy.ini (and other config files): -
• Python version used: 3.13.7

[FelixSchwarz]

@provinzkraut I can only reproduce the described behavior with number of non-default parameters:
mypy --no-namespace-packages --warn-unused-ignores --ignore-missing-imports file.py

Could you check your mypy.ini?

[provinzkraut]

Thanks @FelixSchwarz, you're right, I missed those. I'll update the description

[FelixSchwarz]

to be clear --ignore-missing-imports is not necessary but mypy will yell at you if that one is missing.

[sterliakov]

Okay, this is a bit weird - why are you running with --no-namespace-packages when opentelemetry IS a namespace package? It's still wrong that results differ with and without cache, but why are you doing that?..

Furthermore, this has changed between 1.18.2 and master - 1.18.2 loops as you report, and current HEAD reports unused-ignore on all runs except the first (cold) one. This change bisects to #19820.

[provinzkraut]

Okay, this is a bit weird - why are you running with --no-namespace-packages when opentelemetry IS a namespace package? It's still wrong that results differ with and without cache, but why are you doing that?..

I'm not doing that for this check in particular, it's just what was needed to reproduce it. I've encountered this error in the wild as part of a larger project, where no-namespace-packages is set in the mypy.ini (as pointed out by @FelixSchwarz).

[sterliakov]

Just to confirm, is that larger project also enabling --no-namespace-packages despite actually depending on some namespace packages like opentelemetry? This is clearly a bug anyway, but it feels wrong to disable mod resolution you are actually using.

[provinzkraut]

Just to confirm, is that larger project also enabling --no-namespace-packages despite actually depending on some namespace packages like opentelemetry?

Yes, that is the case. I can't speak as to why this is though. It's an old thing and we're just beginning to renovate its typing.