Support __path__ in packages

[JukkaL]

The name __path__ is defined in packages but mypy doesn't recognize it. It seems that every __init__.py[i] file should have an implicit __path__ attribute with type List[str]. (I encountered some code that uses __path__ -- I didn't know about it until today.)

See also: https://docs.python.org/2/tutorial/modules.html#packages-in-multiple-directories

[tonygrue-dbx]

As a workaround for others (given to me by the experts) you can ignore this issue until it's fixed as such:

__path__.append(...)  # type: ignore  # mypy issue #1422

[gvanrossum]

Shouldn't be too hard to fix?

[rth]

If it helps a minimal reproducible example can be found below (I wrote one before discovering this issue),

A package with the following structure,

pkg1/
  - setup.py
  - mypy_issue_example/
    - __init__.py

with
setup.py

from setuptools import setup, find_packages

setup(
    name='mypy-issue-example',
    version='0.0',
    packages=find_packages(),
)

and
mypy_issue_example/__init__.py

def check_path() -> str:
    import mypy_issue_example
    return mypy_issue_example.__path__

Running,

$ pip install -e .
$ python -c "import mypy_issue_example; print(mypy_issue_example.__path__)"
['[...]/pkg1/mypy_issue_example']
$ mypy mypy_issue_example
mypy_issue_example/__init__.py:4: error: Module has no attribute "__path__"
Found 1 error in 1 file (checked 1 source file)