[GitHub] [beam] damccorm opened a new issue, #19954: Python typehints: support forward references

[GitBox <gi...@apache.org>]

damccorm opened a new issue, #19954:
URL: https://github.com/apache/beam/issues/19954

   Typehints may be given as string literals: https://www.python.org/dev/peps/pep-0484/#forward-references
   These are currently not evaluated and result in errors.
   Example 1:
   ```
   
     def test_typed_callable_string_hints(self):
       def do_fn(element: 'int') -> 'typehints.List[str]':
   
        return [[str(element)] * 2]
   
       result = [1, 2] | beam.ParDo(do_fn)
       self.assertEqual([['1',
   '1'], ['2', '2']], sorted(result))
   
   ```
   
   This results in:
   ```
   
   >     return issubclass(sub, base)
   E     TypeError: issubclass() arg 2 must be a class or tuple of
   classes
   
   typehints.py:1168: TypeError
   
   ```
   
   
   Example 2:
   ```
   
     def test_typed_dofn_string_hints(self):
       class MyDoFn(beam.DoFn):
         def process(self, element:
   'int') -> 'typehints.List[str]':
           return [[str(element)] * 2]
   
       result = [1, 2] | beam.ParDo(MyDoFn())
   
      self.assertEqual([['1', '1'], ['2', '2']], sorted(result))
   
   ```
   
   This results in:
   ```
   
   >     raise ValueError('%s is not iterable' % type_hint)
   E     ValueError: typehints.List[str] is
   not iterable
   
   typehints.py:1194: ValueError
   
   ```
   
   where the non-iterable entity the error refers to is a string literal ("typehints.List[str]").
   
   
   Imported from Jira [BEAM-8487](https://issues.apache.org/jira/browse/BEAM-8487). Original Jira may contain additional context.
   Reported by: udim.


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