[GitHub] [beam] kennknowles opened a new issue, #18957: Element type inference doesn't work for multi-output DoFns

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

kennknowles opened a new issue, #18957:
URL: https://github.com/apache/beam/issues/18957

   TLDR: if you have a multi-output DoFn, then the non-main PCollections with incorrectly have their element types set to None. This affects type checking for pipelines involving these PCollections.
   
   Minimal example:
   ```
   
   import apache_beam as beam
   
   class TripleDoFn(beam.DoFn):
     def process(self, elem):
       yield_elem
   
      if elem % 2 == 0:
         yield beam.pvalue.TaggedOutput('ten_times', elem * 10)
       if elem % 3
   == 0:
         yield beam.pvalue.TaggedOutput('hundred_times', elem * 100)
         
   @beam.typehints.with_input_types(int)
   @beam.typehints.with_output_types(int)
   class
   MultiplyBy(beam.DoFn):
     def __init__(self, multiplier):
       self._multiplier = multiplier
   
     def
   process(self, elem):
       return elem * self._multiplier
     
   def main():
     with beam.Pipeline() as
   p:
       x, a, b = (
         p
         | 'Create' >> beam.Create([1, 2, 3])
         | 'TripleDo' >> beam.ParDo(TripleDoFn()).with_outputs(
   
          'ten_times', 'hundred_times', main='main_output'))
   
       _ = a | 'MultiplyBy2' >> beam.ParDo(MultiplyBy(2))
   
   if
   __name__ == '__main__':
     main()    
   
   ```
   
   Running this yields the following error:
   ```
   
   apache_beam.typehints.decorators.TypeCheckError: Type hint violation for 'MultiplyBy2': requires <type
   'int'> but got None for elem
   
   ```
   
   Replacing `a` with `b` yields the same error. Replacing `a` with `x` instead yields the following error:
   ```
   
   apache_beam.typehints.decorators.TypeCheckError: Type hint violation for 'MultiplyBy2': requires <type
   'int'> but got Union[TaggedOutput, int] for elem
   
   ```
   
   I would expect Beam to correctly infer that `a` and `b` have element types of `int` rather than `None`, and I would also expect Beam to correctly figure out that the element types of `x` are compatible with `int`.
   
   Imported from Jira [BEAM-4132](https://issues.apache.org/jira/browse/BEAM-4132). Original Jira may contain additional context.
   Reported by: chuanyu.


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