Add support for kw_only dataclasses (addresses #36978)

[assaf127]

Add support for dataclasses with kw_only=True, and for dataclasses with init=False fields.
This change encode the dataclass field names as well as their values in order to pass them to the dataclass __init__ function by-keyword on decoding.

fixes #36978

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[gemini-code-assist]

Summary of Changes

Hello @assaf127, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the Apache Beam Python SDK's coding capabilities to support modern Python dataclass features, specifically kw_only=True and fields with init=False. By modifying how dataclass fields are encoded and decoded, the system can now correctly serialize and deserialize these more complex dataclass structures, ensuring data integrity and compatibility across Beam pipelines.

Highlights

• Dataclass Encoding: The encoding mechanism for dataclasses has been updated to explicitly encode field names along with their values. This change specifically targets fields where init=True, ensuring that only relevant fields are serialized.
• Dataclass Decoding: The decoding process for dataclasses now reconstructs instances using keyword arguments. This allows for proper deserialization of dataclasses defined with kw_only=True, as well as handling fields with init=False by only considering fields that were part of the initialization.
• Test Coverage: New test cases have been introduced to validate the encoding and decoding of kw_only dataclasses. This includes a new FrozenKwOnlyDataClass and checks within both deterministic and cross-process encoding tests.

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[codecov]

Codecov Report

❌ Patch coverage is 31.57895% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.36%. Comparing base (0f7a189) to head (b9ad291).
⚠️ Report is 93 commits behind head on master.

Files with missing lines	Patch %	Lines
sdks/python/apache_beam/coders/coder_impl.py	14.28%	12 Missing ⚠️
...ks/python/apache_beam/coders/coders_test_common.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #36979      +/-   ##
============================================
- Coverage     40.38%   40.36%   -0.02%     
  Complexity     3470     3470              
============================================
  Files          1226     1226              
  Lines        188337   188418      +81     
  Branches       3603     3603              
============================================
+ Hits          76053    76063      +10     
- Misses       108883   108954      +71     
  Partials       3401     3401              

Flag	Coverage Δ
python	40.62% <31.57%> (-0.03%)	⬇️

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[github-actions]

Checks are failing. Will not request review until checks are succeeding. If you'd like to override that behavior, comment assign set of reviewers

[assaf127]

The failing checks seem irrelevant for this PR.
Also, the coverage report doesn't seems right - I specifically added tests that run over the lines that appear uncovered. Perhaps it doesn't count lines that are run in a subprocess?

[assaf127]

assign set of reviewers

[github-actions]

Assigning reviewers:

R: @damccorm for label python.

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[damccorm]

from the lint check:

apache_beam/coders/coder_impl.py:387: error: Incompatible types in assignment (expression has type "Callable[[Any], bool]", variable has type "_lru_cache_wrapper[Any]")  [assignment]

Could you please take a look?

[damccorm]

I'll let checks run, but other than linting this generally looks good to me. Thanks!

[damccorm]

/gemini review

[gemini-code-assist]

Code Review

This pull request adds support for encoding and decoding dataclasses that use kw_only=True. The changes correctly handle keyword-only fields by encoding field names along with values, and also fix a bug where fields with init=False were being incorrectly encoded. The implementation is sound and the tests are thorough. I've suggested a couple of minor refactorings to improve code clarity and reduce duplication.

[assaf127]

from the lint check:

apache_beam/coders/coder_impl.py:387: error: Incompatible types in assignment (expression has type "Callable[[Any], bool]", variable has type "_lru_cache_wrapper[Any]")  [assignment]

Could you please take a look?

Should be fixed now, thanks!

[damccorm]

Thanks!