[GitHub] [beam] kennknowles opened a new issue, #19288: Sources using Metrics.counter fail with 'Can't pickle ... DelegatingCounter'

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kennknowles opened a new issue, #19288:
URL: https://github.com/apache/beam/issues/19288

   This fails under Python 2.7 but works fine under Python 3 – looks like an easy fix.
   
   I am reading from a beam.io.utils.CountingSource:
   
   `beam.io.Read(beam_io_utils.CountingSource(NUM_ROWS))`
   
   This fails under python 2.7 with:
   
   Python exception: Can't pickle <class 'apache_beam.metrics.metric.DelegatingCounter'\>: attribute lookup apache_beam.metrics.metric.DelegatingCounter failed
   
   Looks like this is because DelegatingCounter is actually an inner class defined inside apache_beam.metrics.metric.Metric. Under Python 3 pickling can resolve it under its qualified name apache_beam.metrics.metric.Metric.DelegatingCounter, but not in Python 2. A fix would be to move it to a top-level class in that module.
   
   This also applies to other sources which use Metrics.counter.
   
   Imported from Jira [BEAM-6389](https://issues.apache.org/jira/browse/BEAM-6389). Original Jira may contain additional context.
   Reported by: matthjw.


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