Named tuples in import cycles still fail sometimes

[ilevkivskyi]

It looks like issue #3054 was not completely fixed. Namely, in some scenarios named tuples
in import cycles result in Invalid type. For example, checking this file

import posix

results in

typeshed/stdlib/3/os/__init__.pyi:331: error: Invalid type "posix.uname_result"
typeshed/stdlib/3/os/__init__.pyi:599: error: Invalid type "posix.times_result"
typeshed/stdlib/3/os/__init__.pyi:605: error: Invalid type "posix.waitid_result"
typeshed/stdlib/3/os/__init__.pyi:622: error: Invalid type "posix.sched_param"
typeshed/stdlib/3/os/__init__.pyi:624: error: Invalid type "posix.sched_param"
typeshed/stdlib/3/os/__init__.pyi:625: error: Invalid type "posix.sched_param"

Note that the above "invalid" types are all named tuples and there is an import cycle os <-> posix. Interestingly, import os and from os import * both typecheck cleanly.

I haven't yet tried to find a simple repro (not involving typeshed).

[ilevkivskyi]

Somehow this breaks typshed build since recently, so I raised priority to high.

[elliott-beach]

I deleted code from the typestub until I got, in posix.pyi,

from os import stat_result

# This was NamedTuple('etc'), but that doesn't affect the error.
uname_result = "dummy" # E: Invalid type "posix.uname_result" 

and in os/__init__.pyi,

stat_result = "dummy"
from posix import T
def uname() -> uname_result: ...

so there is something fishy with the way circular imports are resolved, but it doesn't seem to me to involve NamedTuple specifically.

[elliott-beach]

Actually, I think this is just a case of the bad circular import discussed in https://stackoverflow.com/a/40094439/5749914 (posix.py is Y.py, os/__init__.py is X.py)

Mypy executes from os import stat_result, but then proceeds to import the rest of os/__init__.pyi, resulting in an error when uname_result is undefined.

[gvanrossum]

Do note that circular dependencies at run-time work differently than in mypy's analysis.

[elliott-beach]

Yes, but if it wouldn't work by python's runtime, I figure there is no reason to expect it to work for mypy, at least in this case IMO.