DoFn.process should raise exception if something other than a List is returned

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. Original Jira may contain additional context. Reported by: chuanyu.