pytype hangs on alias from another module inside class

[srittau]

Please consider the following stub file:

import unittest.mock

class MockFixture:
    Mock = unittest.mock.Mock

When checking this with pytype (2020.06.01), pytype hangs indefinitely, while consuming 100% CPU time.

[rchen152]

I believe we currently only allow aliases inside a class to another member of the same class, due to some parser limitations. But the parser usually prints a sensible error message instead of hanging forever, so at the least that should be fixed.

[srittau]

I discovered this limitation while working on python/typeshed#4275. One temporary solution would be if pytype would just treating those aliases as Any. This would allow type stubs with those aliases to work at least. But I am not sure how complex this would be to implement. This discussion is probably out of scope for this issue, though.

[rchen152]

Rewriting

class Foo:
  Alias = SomeType

as

from typing import Type
class Foo:
  Alias: Type[SomeType]

seems to work. (which leads to the question: could pytype just automatically do this rewrite behind the scenes?)

[JelleZijlstra]

Not sure if pytype has more permissive rules around Type, but I don't think that would fix all issues in pytest_mock. MockFixture.mock_open is a function, for example (an alias of unittest.mock.mock_open), and I don't think you're allowed to Type[] a function.

[Akuli]

I ran into possibly the same problem with a class from the same module. Minimal repro (.pyi file)

class Foo:
    def meth(self) -> None: ...

class Bar:
    meth = Foo.meth

When trying to resolve this aliasing, pytype gets stuck into this infinite while loop in pytype/pyi/parser.py:

        while isinstance(val, pytd.Alias):
          if isinstance(val.type, pytd.NamedType):
            _, _, base_name = val.type.name.rpartition(".")
            if base_name in vals_dict:
              val = vals_dict[base_name]
              continue
          raise ParseError(
              "Illegal value for alias %r. Value must be an attribute "
              "on the same class." % val.name)

I haven't tried to figure out what this loop is supposed to do yet so I don't know how to fix this.

Being able to do this kind of aliasing would allow me to get rid of some copy/pasta in stub files that I'm currently working on.

[rchen152]

Thanks for tracking that down! I think I'll try just putting in a type of Any when a loop is detected - beats hanging forever.

[Akuli]

What is currently the preferred way to say "Bar.meth has the same arguments and return types as Foo.meth"? This doesn't seem to work:

class Foo:
    def meth(self) -> None: ...

class Bar:
    meth: Type[Foo.meth]

Edit: If the Any workaround will be released in a few days then nevermind.

[rchen152]

pytype 2020.07.24 contains a workaround that allows arbitrary aliases as class attributes; let me know if something isn't working.