Warn when DoFn.process uses both yield and return

[liferoad]

addresses #22969:

Do not allow mixing yield and return statements in the process method for DoFn

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[liferoad]

@tvalentyn PTAL.

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Codecov Report

Merging #25743 (b979176) into master (d9f81b8) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #25743      +/-   ##
==========================================
- Coverage   71.44%   71.42%   -0.03%     
==========================================
  Files         777      777              
  Lines      102275   102395     +120     
==========================================
+ Hits        73075    73140      +65     
- Misses      27745    27800      +55     
  Partials     1455     1455              

Flag	Coverage Δ
python	79.96% <100.00%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
sdks/python/apache_beam/transforms/core.py	93.14% <100.00%> (+0.22%)	⬆️

... and 17 files with indirect coverage changes

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[liferoad]

Run PythonDocker PreCommit

[shunping]

Just noticed that the dofn used by PeriodicImpulse was also flagged for this:

beam/sdks/python/apache_beam/transforms/periodicsequence.py

Line 172 in 423a3c3

return

WARNING:apache_beam.transforms.core:Using yield and return in the process method of <class 'apache_beam.transforms.periodicsequence.ImpulseSeqGenDoFn'> can lead to unexpected behavior, see:https://github.com/apache/beam/issues/22969.

[tvalentyn]

I think it is harmless since we are not returning anything, but #37153 should remove the warning.

[shunping]

Found another one:)

WARNING:apache_beam.transforms.core:Using yield and return in the process method of <class 'apache_beam.io.fileio._ReadMatchesFn'> can lead to unexpected behavior, see:https://github.com/apache/beam/issues/22969.

beam/sdks/python/apache_beam/io/fileio.py

Line 233 in ca27526

class _ReadMatchesFn(beam.DoFn):

This looks like harmless as the previous one.

Maybe it is better to only give a warning if yield and return in the same function AND some of the returns are returning something?