Daemonized mypy ignores errors on files that previously passed (cache bug)

[emernic]

Bug Report
dmypy is failing to notice type errors when they are introduced into a file that has already passed type checking. I'm observing this even on extremely minimal examples. It seems like once a file passes checks once it is never checked again by dmypy.

Specifically, this seems to happen when dmypy is given a package. Explicitly passing the filename still gives the expected errors.

To Reproduce
Create a package called test_package containing an empty __init__.py and a file called test.py with the contents:

a: int = 1
b: int = "foo"

Run the following commands:

$ dmypy run -- -p test_package
test_package/test.py:2: error: Incompatible types in assignment (expression has type "str", variable has type "int")  [assignment]
Found 1 error in 1 file (checked 2 source files)

# Comment out line 2 of test.py

$ dmypy run -- -p test_package
Success: no issues found in 2 source files

# Uncomment line 2 of test.py

$ dmypy run -- -p test_package
Success: no issues found in 2 source files

Expected Behavior
The last command should raise the "Incompatible types in assignment" error.

Actual Behavior
No issues are found, despite the contents of test.py being exactly the same as the first run.

Your Environment

• Mypy version used: 1.13.0
• Mypy command-line flags: (see examples above)
• Mypy configuration options from mypy.ini (and other config files): None
• Python version used: 3.11.6

[brianschubert]

Thanks for the report!

I'm having some trouble reproducing this locally. After uncommenting the second line in test_package/test.py and re-running dmypy run -- -p test_package, I'm getting the original error as expected.

Can you think of any other details that might help narrow this down?

[emernic]

I just tried and failed reproduce it on another machine running the same Python and mypy versions, so it's definitely environment-specific. There's a lot going on in the environment I'm seeing the error in (it's a GitHub Codespace with git, pre-commit, poetry, and a lot of other stuff) so I might not have time to isolate the determining factor for quite a while. Feel free to close.

[hauntsaninja]

Could be the same issue as #15677 (comment)