Warn about unused `type: ignore` comments when error code is disabled by brianschubert · Pull Request #18849 · python/mypy

brianschubert · 2025-03-28T18:22:36Z

hauntsaninja · 2025-03-29T02:17:28Z

The primer hits aren't obvious to me, but maybe I'm missing something?

hauntsaninja · 2025-03-29T02:18:32Z

test-data/unit/check-errorcodes.test

+[case testErrorCodeWarnUnusedIgnores7_WarnWhenErrorCodeDisabled]
+# flags: --warn-unused-ignores --disable-error-code name-defined
+x              # type: ignore[name-defined]                # E: Unused "type: ignore" comment  [unused-ignore]
+"x".foobar(y)  # type: ignore[name-defined, attr-defined]  # E: Unused "type: ignore[name-defined]" comment  [unused-ignore]


maybe also check interactions with # type: ignore[unused-ignore]?

Added some tests! The behavior seems to be what we'd want - adding # type: ignore[unused-ignore] suppresses any unused-ignore warnings. There's no way to make mypy warn about an unused # type: ignore[unused-ignore], but I don't think that's a problem?

github-actions · 2025-03-30T21:42:30Z

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
+ scipy-stubs/sparse/_lil.pyi:149: error: Unused "type: ignore" comment  [unused-ignore]
+ scipy-stubs/sparse/_lil.pyi:167: error: Unused "type: ignore" comment  [unused-ignore]
+ scipy-stubs/sparse/_csr.pyi:28: error: Unused "type: ignore" comment  [unused-ignore]
+ scipy-stubs/sparse/_csr.pyi:41: error: Unused "type: ignore" comment  [unused-ignore]
+ scipy-stubs/sparse/_csc.pyi:30: error: Unused "type: ignore" comment  [unused-ignore]
+ scipy-stubs/sparse/_csc.pyi:39: error: Unused "type: ignore" comment  [unused-ignore]

prefect (https://github.com/PrefectHQ/prefect)
+ src/prefect/server/orchestration/core_policy.py:418: error: Unused "type: ignore" comment  [unused-ignore]
+ src/prefect/server/orchestration/core_policy.py:486: error: Unused "type: ignore" comment  [unused-ignore]

static-frame (https://github.com/static-frame/static-frame)
+ static_frame/core/index.py:1509: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/index_datetime.py:152: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/index_datetime.py:304: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/index_hierarchy.py:1034: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/frame.py:3976: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/frame.py:5610: error: Unused "type: ignore" comment  [unused-ignore]
+ static_frame/core/frame.py:6199: error: Unused "type: ignore" comment  [unused-ignore]

mitmproxy (https://github.com/mitmproxy/mitmproxy)
+ mitmproxy/coretypes/multidict.py:197: error: Unused "type: ignore" comment  [unused-ignore]
+ mitmproxy/proxy/layers/quic/_raw_layers.py:46: error: Unused "type: ignore" comment  [unused-ignore]

kornia (https://github.com/kornia/kornia)
+ kornia/augmentation/auto/base.py:44: error: Unused "type: ignore" comment  [unused-ignore]

werkzeug (https://github.com/pallets/werkzeug)
+ src/werkzeug/exceptions.py:208: error: Unused "type: ignore" comment  [unused-ignore]

sympy (https://github.com/sympy/sympy)
+ sympy/solvers/ode/single.py:1745: error: Unused "type: ignore" comment  [unused-ignore]

dedupe (https://github.com/dedupeio/dedupe)
+ dedupe/predicates.py:120: error: Unused "type: ignore" comment  [unused-ignore]

materialize (https://github.com/MaterializeInc/materialize)
+ misc/python/materialize/mzexplore/common.py:93: error: Unused "type: ignore" comment  [unused-ignore]

pwndbg (https://github.com/pwndbg/pwndbg)
+ pwndbg/aglib/disasm/__init__.py:92: error: Unused "type: ignore" comment  [unused-ignore]
+ pwndbg/aglib/heap/ptmalloc.py:144: error: Unused "type: ignore" comment  [unused-ignore]

jax (https://github.com/google/jax)
+ jax/experimental/mosaic/gpu/inference_utils.py:35: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/experimental/mosaic/gpu/inference_utils.py:46: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/experimental/mosaic/gpu/inference_utils.py:57: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/experimental/mosaic/gpu/inference_utils.py:68: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/experimental/mosaic/gpu/inference_utils.py:75: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/experimental/mosaic/gpu/inference_utils.py:111: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/traceback_util.py:134: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/sharding_specs.py:147: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/pretty_printer.py:54: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/sharding.py:203: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/named_sharding.py:201: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/linear_util.py:470: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/linear_util.py:471: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/core.py:1673: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/sharding_impls.py:177: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/array.py:784: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/interpreters/pxla.py:1637: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/export/_export.py:689: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/_src/export/_export.py:774: error: Unused "type: ignore" comment  [unused-ignore]
+ jax/experimental/jax2tf/jax2tf.py:1424: error: Unused "type: ignore" comment  [unused-ignore]

asynq (https://github.com/quora/asynq)
+ asynq/tools.pyi:89: error: Unused "type: ignore" comment  [unused-ignore]
+ asynq/tools.pyi:91: error: Unused "type: ignore" comment  [unused-ignore]

brianschubert · 2025-03-30T21:45:09Z

I did some analysis on the primer hits:

scipy-stubs: all # type: ignore[explicit-override] comments. Their presence seems to come from them using basedmypy, which I think enables this rule by default? I don't see explicit-override enabled in their pyproject.toml. In any case, I cloned the repo, made a venv to match the mypy_primer.projects definition, ran mypy master (4b1a255) , and got no errors. If I remove the # type: ignore[explicit-override] comments, I still get no errors. So, this seems to be a true positive, the # type: ignore isn't necessary from mypy's perspective. If I add --enable-error-code explicit-override, I do get explicit-override errors on the affected lines.
prefect, static-frame, mitmproxy, kornia, werkzeug, dedupe, materialize, asynq: all bare # type: ignore comments. Cloned each repo, removed the # type: ignore comments, ran mypy master, and got no errors on the affected lines. These all seem to be true positives. I didn't check what disabled error codes might be at play / why these # type: ignore comments were added in the first place.
pwndbg: one # type: ignore[attr-defined], one # type: ignore[assignment]. These error codes are indeed disabled for their respective modules in pyproject.toml, so the # type: ignore comments are unnecessary. Removing them and running mypy master produced no errors on the affected lines. true positive.
jax: only looked at the errors in jax/experimental/mosaic/gpu/inference_utils.py, all were bare # type: ignores. Removing these and running mypy master produced no errors on the affected lines. true positive (plus some unchecked)
sympy: not sure about this one. When I run this changeset locally, I don't get the unused-ignore error. I wasn't able to figure out what's different about my environment

sterliakov · 2025-03-31T00:42:06Z

Re sympy: that seems to depend on how you invoke mypy.

$ mypy sympy --warn-unused-ignores --cache-dir /dev/null | grep ode/single
sympy/solvers/ode/single.py:1745: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:2661: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:2662: error: Unused "type: ignore" comment  [unused-ignore]

$ mypy sympy/solvers/ode/single.py --warn-unused-ignores --cache-dir /dev/null | grep ode/single
sympy/solvers/ode/single.py:22: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:1679: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:2661: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:2662: error: Unused "type: ignore" comment  [unused-ignore]

If you did the latter, unused-ignore doesn't show up.

And with 1.15.0 (compiled):

$ mypy sympy --warn-unused-ignores --cache-dir /dev/null | grep ode/single
sympy/solvers/ode/single.py:2661: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:2662: error: Unused "type: ignore" comment  [unused-ignore]

$ mypy sympy/solvers/ode/single.py --warn-unused-ignores --cache-dir /dev/null | grep ode/single
sympy/solvers/ode/single.py:22: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:1679: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:2661: error: Unused "type: ignore" comment  [unused-ignore]
sympy/solvers/ode/single.py:2662: error: Unused "type: ignore" comment  [unused-ignore]

(and that's a true positive - what they wanted to ignore was [truthy-bool] which isn't even part of --strict AFAIC, they don't have it enabled explicitly anywhere)

brianschubert · 2025-03-31T01:08:26Z

That's interesting! Yup, that seems to be the case. I was using a variant of mypy sympy/solvers/ode/single.py. Using mypy sympy instead, the unused-ignore error shows up, and I get no error when the # type: ignore comment is removed.

Warn about unused type: ignore comments when error code is disabled

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hauntsaninja reviewed Mar 29, 2025

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Add test cases for "type: ignore[unused-ignore]" interaction

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sterliakov approved these changes Mar 31, 2025

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hauntsaninja approved these changes Apr 3, 2025

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hauntsaninja merged commit 6b68661 into python:master Apr 3, 2025
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asottile-sentry mentioned this pull request Jun 4, 2025

1.16 regression: --warn-unused-ignore now complains in ignored-but-correct places #19237

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markfinal mentioned this pull request Jul 7, 2025

mypy warning fixes markfinal/cruiz#249

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Warn about unused `type: ignore` comments when error code is disabled#18849

Warn about unused `type: ignore` comments when error code is disabled#18849
hauntsaninja merged 2 commits intopython:masterfrom
brianschubert:gh-11059-warn-type-ignore-disabled

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