[jira] [Created] (BEAM-4132) Element type inference doesn't work for multi-output DoFns

["Chuan Yu Foo (JIRA)" <ji...@apache.org>]

Chuan Yu Foo created BEAM-4132:
----------------------------------

             Summary: Element type inference doesn't work for multi-output DoFns
                 Key: BEAM-4132
                 URL: https://issues.apache.org/jira/browse/BEAM-4132
             Project: Beam
          Issue Type: Bug
          Components: sdk-py-core
    Affects Versions: 2.4.0
            Reporter: Chuan Yu Foo
            Assignee: Ahmet Altay


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:
{code:python}
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()    
{code}

Running this yields the following error:
{noformat}
apache_beam.typehints.decorators.TypeCheckError: Type hint violation for 'MultiplyBy2': requires <type 'int'> but got None for elem
{noformat}

Replacing {{a}} with {{b}} as follows yields the same error. Replacing {{a}} with {{x}} instead yields the following error:

{noformat}
apache_beam.typehints.decorators.TypeCheckError: Type hint violation for 'MultiplyBy2': requires <type 'int'> but got Union[TaggedOutput, int] for elem
{noformat}

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}}.



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