Importing a new file within a module using "from x import y" fails in incremental mode

[Michael0x2a]

Consider the following test case:

[case testIncrementalFoo]
from parent import a

[file parent/__init__.py]

[file parent/a.py]

[file parent/a.py.next]
from parent import b

[file parent/b.py.next]

[stale parent.a, parent.b]
[out]

It initially starts with two files (parent/__init__.py and parent/a.py) that are both empty, then runs mypy -i -c "from parent import a" to warm up the cache.

We then add a new empty file, parent/b.py, and change parent/a.py to contain the single line from parent import b and re-run mypy -i -c "from parent import a".

The expected behavior is that mypy should typecheck the modified files without complaining, but will instead report the following error:

<string>:1: note: In module imported here:
parent/a.py:1: error: Module has no attribute 'b'

This error disappears when we change parent/a.py to contain either the line import parent.b or import parent.b as b, or if the file parent/b.py existed before mypy was first run.

[gvanrossum]

Good find! I suspect the key to this issue is the ambiguity inherent in `from parent import b`. This can either import an attribute defined in `parent/__init__.py` or a submodule `parent/b.py`. I expect that if you make some dummy change to `__init__.py` to force it to change that will also make the problem go away -- if that's so, the solution probably involves throwing away the cache data for `parent/__init__.py` when a module is added (or removed).

[Michael0x2a]

It seems like making the dummy change does make the problem go away, as you predicted :)

I'll look into the caching logic next then.