Failure to resolve a specific import structure

[elprans]

Given the following module structure:

top
|-- __init__.py
|-- b
|   |-- __init__.py
|   `-- _b.py
`-- c.py

And the following module contents:

• top/__init__.py:

from .b import *
from .c import *

• top/b/__init__.py:

from ._b import *

• top/b/_b.py:

import top
class Cls:
    pass

• top/c.py:

from . import b
foo: b.Cls = None

mypy -m top fails to resolve b.Cls:

top/c.py:2: error: Name 'b.Cls' is not defined

[gvanrossum]

What does mypy --dump-graph -m top print?

[elprans]

[["n0", 91875, ["_importlib_modulespec", "mypy_extensions", "types", "abc", "typing", "builtins", "collections", "sys", "collections.abc"],
     {"_importlib_modulespec": 1810, "mypy_extensions": 731, "types": 7066, "abc": 809, "typing": 18315, "builtins": 43416, "collections": 13579, "sys": 5019, "collections.abc": 1130},
     {}],
 ["n1", 120, ["top.b._b", "top.c", "top.b", "top"],
     {"top.b._b": 33, "top.c": 35, "top.b": 18, "top": 34},
     {}]
]

[gvanrossum]

So from that I learn it's an import cycle problem, where c is processed before b. A workaround may be to change c to say "from b import Cls".

[elprans]

Yes, that workaround would work in this case.

That said, python itself has no problem with cycles like this. You can import a module which is currently being loaded, you're just not guaranteed that it'll have all the attributes you want.

More specifically, an import of top from top.b._b should be short-circuited because top itself has not been fully loaded yet. importlib just does return sys.modules['top'] in such case.

[gvanrossum]

Mypy sometimes gets this right too, but not in all cases, because it does not emulate the precise loading order of modules.