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

["ASF GitHub Bot (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/BEAM-12920?focusedWorklogId=723807&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-723807 ]

ASF GitHub Bot logged work on BEAM-12920:
-----------------------------------------

                Author: ASF GitHub Bot
            Created on: 09/Feb/22 17:05
            Start Date: 09/Feb/22 17:05
    Worklog Time Spent: 10m 
      Work Description: tvalentyn opened a new pull request #16791:
URL: https://github.com/apache/beam/pull/16791


   Assume that bare generator types define simple generators.
   The only type of generators that beam currently handles are simple generators: https://github.com/apache/beam/blob/d7ee6b8c8b4fedb05d31d6554ab55a6d4bca0356/sdks/python/apache_beam/typehints/typehints.py#L1120
   
   This test started to fail on Python 3.9 because bare Generator instances on Python <3.9 were not truly 'bare' and still had __args__ defined, we defined inferred a hint for bare geneators and didn't fail. Preserving this behavior for Python 3.9+ as well. 


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Issue Time Tracking
-------------------

            Worklog Id:     (was: 723807)
    Remaining Estimate: 0h
            Time Spent: 10m

> 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: Sub-task
>          Components: sdk-py-core
>    Affects Versions: 2.32.0
>         Environment: Python 3.9.7
> Linux Mint 20.2, Kernel: 5.4.0-84-generic
>            Reporter: Jonathan Hourany
>            Assignee: Valentyn Tymofieiev
>            Priority: P2
>             Fix For: Not applicable
>
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> In Python 3.9 the test [test_convert_bare_types|https://github.com/apache/beam/blob/6c1c19d6c711babf5f5f042fa465be20948caff6/sdks/python/apache_beam/typehints/native_type_compatibility.py#L40] 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|https://github.com/apache/beam/blob/6c1c19d6c711babf5f5f042fa465be20948caff6/sdks/python/apache_beam/typehints/native_type_compatibility.py#L40] which attempts to access a type's ___args___ attribute. In Python 3.9 bare generators no longer instantiate with a default ___args___ attribute. Only when instantiated with args do they get one (see example 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.
> h3. 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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