Mypy does not recheck files when follow_imports changes for dependencies

[nickgaya]

Steps to reproduce

1. Create module test.py with import from nonexistent module:

from foo import bar

2. Create mypy.ini file:

[mypy]

[mypy-foo]
ignore_missing_imports = True 

3. Run mypy

$ mypy test.py 
Success: no issues found in 1 source file

4. Comment out [mypy-foo] section in mypy.ini

[mypy]

# [mypy-foo]
# ignore_missing_imports = True 

5. Run mypy

$ mypy test.py 
Success: no issues found in 1 source file

Expected result

After step 4, mypy should detect config change and (partially?) invalidate cache, leading to a type error.

$ mypy test.py 
test.py:1: error: Cannot find module named 'foo'
test.py:1: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
Found 1 error in 1 file (checked 1 source file)

Notes

• Deleting .mypy_cache/ and re-running mypy fixes the issue.
• Adding/removing ignore_missing_imports in the main [mypy] section does seem to invalidate the cache as expected. So this issue only seems to apply to [mypy-PATTERN] sections.

Versions

$ python --version
Python 3.7.3

$ mypy --version
mypy 0.740

[msullivan]

Oh this is nasty actually.

In general we recheck a module when its flags change, but since follow_imports is set on the module being imported, changes to that won't cause rechecks.

Probably the simplest solution is that the full set of follow_imports configurations be considered global configuration whose change invalidates everything, though there might be simpler solutions I'm not immediately thinking of. It is also probably possible to do something more targeted that only affects modules that import an affected module?

[nickgaya]

From skimming the code a bit it looks like the cache tracks "suppressed" imports for each module, so maybe you could search through this to find which modules need to be rechecked?