kennknowles opened a new issue, #18712:
URL: https://github.com/apache/beam/issues/18712
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.
Imported from Jira
[BEAM-3530](https://issues.apache.org/jira/browse/BEAM-3530). Original Jira may
contain additional context.
Reported by: chuanyu.
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