cached runs of dmypy stop reporting "unused type: ignore" warnings

Using mypy 0.782, I run
dmypy run .

on my codebase. The first time it runs, it correctly reports a few

error: unused 'type: ignore' comment

same as mypy. However, when rerunning dmypy run ., these errors disappear, while all other errors remain.

For some reason, this type of warning (warn_unused_ignores) seems to be missed sometimes when using the cache.

Unfortunately, it doesn't seem trivial to reproduce, so I can't give a reproducible project right now...

I noticed this today with mypy 0.942, so this is still an issue.

> dmypy run
Success: no issues found in 604 source files

> mypy --cache-dir=/dev/null
apps/matchmaking/services.py:260: error: Unused "type: ignore" comment
Found 1 error in 1 file (checked 604 source files)

> mypy --version
mypy 0.942

This is still a problem in 0.991

Has anybody managed to find a reproducible example yet?

I have recently encountered this on 1.0.1. I have not yet produced a minimal example.

In my case the bulk of the unused type: ignore warnings which disappear are on lines that have been typed by the django-stubs project. I wonder if it may have something to do with that project, or with plugins in general.

I have a new example that doesn't have any external requirements.

unused/__init__.py
(empty file)

unused/empty.py
(empty file)

example.py

from unused.empty import *
a = 1  # type: ignore

Instructions (in a clean virtualenv)

$ dmypy stop
Daemon stopped

$ dmypy run -- --package example --warn-unused-ignores
Daemon started
example.py:2: error: Unused "type: ignore" comment
Found 1 error in 1 file (checked 46 source files)

$ dmypy run -- --package example --warn-unused-ignores
Success: no issues found in 1 source file

From what I can tell this issue happens when we call fine_grained_manager.update([], to_delete) in dmypy_server.Server.fine_grained_increment_follow_imports.

In this case, to_delete is [('unused', '/.../mypy-9655/unused/__init__.py')].

For some reason, when this update is made, the Unused "type: ignore" comment also goes missing.

Next up: dig into mypy, and work out what's going on.

EDIT: I have created a failing test based on this example, and continue to investigate a fix.

Add test to demonstrate issue python#9655

Prove ignores without error codes also disappear

Prove possibly-undefined errors disappear

Ensure errors persist after dmypy update

If anyone following this thread is able to check if #15043 works for them, I'd appreciate the feedback.

@meshy

@meshy Using #15043, I can confirm that Unused "type: ignore" comment errors seem to appear everytime dmypy is ran, without having to restart the daemon. 👌🏻

Fix disappearing errors when re-running dmypy check

Add a regression test for type ignores in dmypy

I think there is another case where type ignore comments still aren't being reported. Here are a few failing test cases:

[case testUnusedTypeIgnorePreservedAfterChangeInOtherModule1]
# flags: --warn-unused-ignores
[file main.py]
import a

def f() -> None:
    1()
    a = 1  # type: ignore

[file a.py]
[file a.py.2]
x = 1

[out]
main.py:4: error: "int" not callable
main.py:5: error: Unused "type: ignore" comment
==
main.py:4: error: "int" not callable
main.py:5: error: Unused "type: ignore" comment

[case testUnusedTypeIgnorePreservedAfterChangeInOtherModule2]
# flags: --warn-unused-ignores
[file main.py]
import a

def f() -> None:
    1()
    a = 1  # type: ignore

def g() -> None:
    1()  # type: ignore

[file a.py]
[file a.py.2]
x = 1

[out]
main.py:4: error: "int" not callable
main.py:5: error: Unused "type: ignore" comment
==
main.py:4: error: "int" not callable
main.py:5: error: Unused "type: ignore" comment

The error on line 5 isn't reported on the second step.

The root cause is that we don't report unused type ignores in nodes only processed using reprocess_nodes, i.e. when following fine-grained dependencies or reprocessing nodes that previously generated errors.

I think this is a potential way to fix this (but I haven't thought about this very carefully yet):

Collect a set of all the triggered/processed nodes in propagate_changes_using_dependencies.
At the end of a daemon update, perform the equivalent of generate_unused_ignore_notes, but only report errors within the previously triggered nodes. The filtering step seems necessary, as some parts of modules are often not processed during a fine-grained increment.

@JukkaL

@JukkaL good catch.

Do you think this blocks #15043, or could we tackle that case separately?

Ah, I forgot to mention that the test cases I mentioned above aren't specifically covered by #15043 yet. It's okay to leave fixing these for another PR, but we shouldn't close this issue until all known cases are fixed.

OK great, thank you. I've removed the "fixes" comment from that PR so we avoid closing this issue.

Add test to demonstrate issue python#9655

Prove ignores without error codes also disappear

Prove possibly-undefined errors disappear

Fix disappearing errors when re-running dmypy check

Add a regression test for type ignores in dmypy

Type ignore comments erroneously marked as unused by dmypy (#15043)

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cached runs of dmypy stop reporting "unused type: ignore" warnings #9655

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jgillard commented on Mar 30, 2022

peterbygrave commented on Nov 30, 2022

AlexWaygood commented on Nov 30, 2022

meshy commented on Feb 20, 2023

meshy commented on Feb 27, 2023

meshy commented on Mar 3, 2023

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meshy commented on Jan 2, 2024

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JukkaL commented on Feb 5, 2024

meshy commented on Feb 5, 2024

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