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

[rggjan]

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...

[jgillard]

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

[peterbygrave]

This is still a problem in 0.991

[AlexWaygood]

Has anybody managed to find a reproducible example yet?

[meshy]

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.

[meshy]

I have produced a minimal example (see edit at bottom) on Python 3.10.6.

requirements.txt

# These are required for mypy
mypy==1.0.1
typing_extensions==4.4.0
mypy-extensions==0.4.3

# Something in this package causes the problem.
numpy==1.22.2

example.py

import numpy
a = 1  # type: ignore

Instructions (in a clean virtualenv)

$ pip install --no-deps -r requirements.txt
...

$ dmypy stop
Daemon stopped

$ dmypy run -- . --warn-unused-ignores
Daemon started
example.py:2: error: Unused "type: ignore" comment
Found 1 error in 1 file (checked 161 source files)

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

EDIT: In hindsight, this isn't a minimal enough example, it's just the minimum I can get without digging into numpy to work out what's going on in there. I guess that's next!

[meshy]

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.