"Module has no attribute" error with import cycle #2016

The following code generates an unexpected error message:

$ head *
==> part1.py <==
from part3 import part3_thing
class FirstThing: pass

==> part2.py <==
from part4 import part4_thing

from socket import socket as Thing

==> part3.py <==
from part2 import Thing
def part3_thing(x, *y): pass
_thing = Thing()

==> part4.py <==
MYPY = False
if MYPY:
    from part1 import FirstThing
part4_thing = 1

Here's what I get when I try to type check it:

$ mypy part{1,2,3,4}.py
part3.py:1: error: Module has no attribute 'Thing'

If I change the order of modules the error won't occur (part{1,3,2,4}.py), as it depends on the order in which we process the cycle.

This is related to #2015.

A potential fix would be to flag the import in part4 as low priority, resulting in a more consistently correct order of processing of the cycle (see #2015 for a discussion of this).

Another, likely less nice fix would be to run the semantic analysis pass that resolves from ... import ... statements within a cycle multiple times somehow, until all imports have been resolved.