mypy sometimes emits fewer errors when running on multiple files at once

[marc-legendre]

Bug Report

Hello there,

I am witnessing (rare) cases where mypy emits fewer errors when running on multiple files at the same time than when running on each file individually.

More specifically, I noticed that mypy can fail to report an error when an import for a submodule is missing a one file but present in another file.

I failed to find anything relevant in the documentation or in the existing issues, so I thought I'd let you know. Below is my attempt at a minimal reproducible example.

To Reproduce

(Since multiple files are needed, here is an archive with the reproduction case: repro.zip)

Given a module mymodule with nested modules a and b, and this file foo.py:

import mymodule.a


def blah() -> None:
    mymodule.b.fn()


blah()

As expected, when called on foo.py, mypy emits an error:

$ mypy --cache-dir /dev/null foo.py
foo.py:5: error: Module has no attribute "b"
Found 1 error in 1 file (checked 1 source file)

yet if I add to mypy's command line a second file which does import mymodule.b, say bar.py:

import mymodule.b

then mypy does not emit the error any more, even though I would expect it to:

$ mypy --cache-dir /dev/null foo.py bar.py
Success: no issues found in 2 source files

Your Environment

I used mypy 0.991 on Ubuntu 22.04 with python 3.10.6; I can reproduce this behavior with master, as well as python 3.8, and older versions of mypy.