robertwb commented on a change in pull request #11838:
URL: https://github.com/apache/beam/pull/11838#discussion_r437702523



##########
File path: sdks/python/apache_beam/testing/test_stream.py
##########
@@ -291,10 +291,10 @@ def expand(self, pbegin):
     assert isinstance(pbegin, pvalue.PBegin)
     self.pipeline = pbegin.pipeline
     if not self.output_tags:
-      self.output_tags = set([None])
+      self.output_tags = {None}

Review comment:
       If the user explicitly sets the output tags to {None}, they might be 
expecting a dict. (Specifically, they might get a set from elsewhere, and set 
the output tags from that set, and it would be awkward to have to check that 
set to determine how to interpret the result. So in this case I would do
   
   ```
   if not self.output_tags:
     return pvalue.PCollection(self.pipeline, is_bounded=False)
   else:
     return { ... for tag in self.output_tags}
   ```




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