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https://issues.apache.org/jira/browse/BEAM-3530?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kenneth Knowles updated BEAM-3530:
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Issue Type: Improvement (was: Bug)
> DoFn.process should raise exception if something other than a List is returned
> ------------------------------------------------------------------------------
>
> Key: BEAM-3530
> URL: https://issues.apache.org/jira/browse/BEAM-3530
> Project: Beam
> Issue Type: Improvement
> Components: sdk-py-core
> Reporter: Chuan Yu Foo
> Priority: Major
> Labels: starter, triaged
>
> The process method of DoFns can either return values or yield values. In the
> case of returning values, it expects a List of elements to be returned. When
> returning a single value, it is easy to forget this, and return the value
> instead.
> Correct way:
> {{class SomeDoFn(beam.DoFn)}}
> {{ def process(self, elem):}}
> {{ return ['a']}}
> Incorrect way:
> {{class SomeDoFn(beam.DoFn)}}
> {{ def process(self, elem):}}
> {{ return 'a'}}
> A pipeline with the incorrect DoFn will fail will a cryptic error message
> without a direct indication that the actual error is due to SomeDoFn
> returning an element instead of a List containing that element. This issue is
> very time-consuming to track down.
> It would be good if the pipeline could raise an exception or otherwise
> indicate that the DoFn is incorrectly returning an element instead of a List
> to make it easier to identify the error.
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