[jira] [Created] (BEAM-12920) Convert To Bare Types Fails on Generators in Python 3.9

["Jonathan Hourany (Jira)" <ji...@apache.org>]

Jonathan Hourany created BEAM-12920:
---------------------------------------

             Summary: Convert To Bare Types Fails on Generators in Python 3.9
                 Key: BEAM-12920
                 URL: https://issues.apache.org/jira/browse/BEAM-12920
             Project: Beam
          Issue Type: Bug
          Components: test-failures
    Affects Versions: 2.32.0
         Environment: Python 3.9.7
Linux Mint 20.2, Kernel: 5.4.0-84-generic
            Reporter: Jonathan Hourany


In Python 3.9 the test [test_convert_bare_types|] fails when testing the following Generator test case
{code:python}
test_cases += [
          (
              'bare generator',
              typing.Generator,
              typehints.Generator[typehints.TypeVariable('T_co')]),
      ]
{code}
Raising the following {{ValueError}}
{noformat}
./sdks/python/apache_beam/typehints/native_type_compatibility_test.py::NativeTypeCompatibilityTest::test_convert_bare_types Failed: [undefined]ValueError: Unsupported Generator with no arguments.
{noformat}
----
The immediate cause may be stemming from [get_args|] which attempts to access a type's ___args___ attribute. In Python 3.9 bare generators no long instantiate with a default ___args___ attribute by default. Only after being passed args do they get one (see below).

However,  despite it's name the test doesn't create a generator that's totally bare, so the deeper cause may be coming from the way {{typinghints.Generator}} is constructed.

Examples:
{code:python}
# Python 3.7.5
In [1]: from typing import Generator
----
In [2]: Generator.__args__
Out[2]: (+T_co, -T_contra, +V_co)
{code}
----

{code:python}
# Python 3.9.7
In [1]: from typing import Generator

In [2]: Generator.__args__
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-2-3e272b948083> in <module>
----> 1 Generator.__args__

~/.pyenv/versions/3.9.7/lib/python3.9/typing.py in __getattr__(self, attr)
    704         if '__origin__' in self.__dict__ and not _is_dunder(attr):
    705             return getattr(self.__origin__, attr)
--> 706         raise AttributeError(attr)
    707 
    708     def __setattr__(self, attr, val):

AttributeError: __args__

In [3]: # Initializing with args, however, creates the needed attr
In [4]: Generator[int, int, int].__args__
Out[4]: (int, int, int)
{code}



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