[YAML] Add Partition transform.

[robertwb]

This allows one to split a single PCollection into multiple distinct PCollections based on some property of the elements.

It performs the role of (numeric) partition and side outputs from other languages.

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R: @Polber

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[lostluck]

I mostly object to calling a Split transform, when it's doing what we've termed largely as Partitioning in beam. Splitting as a Beam concept is generally around Bundles (and increasing Processing Parallelism within a PCollection). It does us no good to muddy the definitions.

So why is it not being called Partition?

[robertwb]

I'm all for trying to find better names for this.

I thought about Partition, but Beam already has a Partition operation that's a bit different: it takes in a integer N and splits the PCollection into N numbered PCollections: https://beam.apache.org/documentation/programming-guide/#partition My other hesitation is that partition is used like shard (and roughly) parallelism of otherwise homogenous datasets in other systems like Kafka. But that's not a blocker.

I really haven't settled on naming that I like here (and went back and forth when writing this). Maybe we should bikeshed on the dev list.

[lostluck]

I'd say while partitions are inherently numerically based, that's largely an implementation detail to serve the user goal of "turn 1 PCollection into N PCollections, known at Pipeline Construction Time".

Personally I'd say that the "standard" beam one is just a "PartitionByIndex", vs a general "PartitionBy" transform that this appears to be.

I have no objection to devlist bikeshedding though.

[robertwb]

After thinking about this some more, you've convinced my Partition is the way to go. Updated.

[robertwb]

Now that the naming is settled, is there anything else that needs to be addressed?

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Codecov Report

Attention: Patch coverage is 92.68293% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 71.44%. Comparing base (65c3e8e) to head (fc480a9).
Report is 25 commits behind head on master.

❗ Current head fc480a9 differs from pull request most recent head 0fe48a2. Consider uploading reports for the commit 0fe48a2 to get more accurate results

Files	Patch %	Lines
sdks/python/apache_beam/yaml/yaml_mapping.py	92.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #30368      +/-   ##
==========================================
- Coverage   71.46%   71.44%   -0.02%     
==========================================
  Files         710      710              
  Lines      104795   104835      +40     
==========================================
+ Hits        74887    74903      +16     
- Misses      28277    28301      +24     
  Partials     1631     1631              

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python	81.24% <92.68%> (-0.03%)	⬇️

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[Polber]

Now that the naming is settled, is there anything else that needs to be addressed?

@robertwb I have the two pending comments above, namely renaming Split to Partition in the docs

[robertwb]

Ah, good catch. Done.