[jira] [Commented] (BEAM-3530) DoFn.process should raise exception if something other than a List is returned

["Juan Carlos Cardenas (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/BEAM-3530?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16585072#comment-16585072 ] 

Juan Carlos Cardenas commented on BEAM-3530:
--------------------------------------------

We added a validation, to check if the result is not iterable or if it is a string:
{code:java}
# validate results is iterable and non-string

try:

validate = iter(results)

except TypeError:

raise TypeError("This is not an iterable")

else:

if isinstance(results, str):

raise TypeError("This shouldn't be string")
{code}

> 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: Bug
>          Components: sdk-py-core
>            Reporter: Chuan Yu Foo
>            Priority: Major
>              Labels: starter
>
> 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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