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From GitBox <...@apache.org>
Subject [GitHub] [beam] chadrik commented on a change in pull request #10822: [BEAM-7746] Minor typing updates / fixes
Date Wed, 26 Feb 2020 23:48:49 GMT
chadrik commented on a change in pull request #10822: [BEAM-7746] Minor typing updates / fixes
URL: https://github.com/apache/beam/pull/10822#discussion_r384836306
 
 

 ##########
 File path: sdks/python/apache_beam/transforms/external_java.py
 ##########
 @@ -37,18 +39,19 @@
 
 # Protect against environments where apitools library is not available.
 # pylint: disable=wrong-import-order, wrong-import-position
+apiclient = None  # type: Optional[types.ModuleType]
 
 Review comment:
   I did some more research on this, and I found this mypy issue: https://github.com/python/mypy/issues/1297
   
   It suggests this idiom:
   
   ```python
   try:
     from apache_beam.runners.dataflow.internal import apiclient as _apiclient
   except ImportError:
     apiclient = None
   else:
     apiclient = _apiclient
   ```
   
   The import is a bit longer and uglier, but it has 2 advantages:
   - no need to import `Optional` or `ModuleType`
   - the idiom I was using was actually making `apiclient` a generic ModuleType, dropping
all knowledge of the members of `apache_beam.runners.dataflow.internal`.  That's bad!
   
   The reason this works without explicit `Optional` annotation that mypy will automatically
determine optionality in some cases, like this:
   
   ```python
   if some_conditional():
     x = None
   else:
     x = 1
   reveal_type(x)  # Revealed type is 'Union[builtins.int, None]'
   ```
   
   
   

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