[xgbtxy]

Fixes #19631.

When every positional parameter has a default, _get_arg_placeholders counts the implicit element as a side-input slot. For process(element=1, timestamp=DoFn.TimestampParam, side_input=3), passing side argument 2 produces (element, 2, timestamp) instead of (element, timestamp, 2).

Skip that first ordinary default in the implicit-element branch, so the existing argument-count and placeholder logic starts at the next parameter. Explicit ElementParam and leading special-marker handling are unchanged. The same helper fixes process_batch; the tests cover that path across the existing FnApiRunner configurations. A Python bugfix note is included in CHANGES.md.

Validation on Windows/Python 3.12 with the native Cython module rebuilt using MSVC:

• All 15 new regression/control executions pass. Before the fix, the two implicit-element positional/side-input cases and seven batch configurations fail; the six explicit/default/keyword controls pass.
• Complete fn_runner_test.py: 465 passed, 81 skipped.
• Complete common, core-transform, PTransform and runtime-typecheck modules with BundleBasedDirectRunner: 283 passed, 3 skipped, 5 failed. Rebuilding the exact original common.py reproduces all five failures in unchanged PTransform tests. The candidate was then restored, rebuilt and its 15 new cases rerun successfully.
• Configured pre-commit checks pass on every changed Python file. Full-repository YAPF passes; Ruff reports the same 30 diagnostics on the original tree and candidate. Full Pyrefly, in the same activated environment, is blocked on the same two generated .pyi UTF-8 decoding errors on both versions. No unrelated lint/type-check changes were made.

The public issue was also reproduced independently on both BundleBasedDirectRunner and FnApiRunner before the change; both now return the expected values. Cloud runners and other Python versions were not run locally. AI assistance: OpenAI Codex.

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[github-actions]

📋 Review Summary

This PR fixes issue #19631 where DoFn.process and DoFn.process_batch methods with a default value on the implicit first (element/batch) parameter misbound subsequent special DoFn parameters and side inputs. By stripping the leading ordinary default in _get_arg_placeholders when all positional parameters have defaults, the element slot and remaining parameter placeholders align properly.

🔍 General Feedback

• The fix in sdks/python/apache_beam/runners/common.py is minimal, well-scoped, and preserves existing handling for explicit DoFn.ElementParam and special parameter markers (_DoFnParam and BundleFinalizerParam).
• Test coverage in common_test.py and fn_runner_test.py thoroughly exercises default, positional, keyword, and side-input bindings for both process and process_batch.

---

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[github-actions]

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[damccorm]

Nit: For a single-parameter def process(self, element=1):, slicing default_arg_values[1:] produces [], so arg_names[-len(default_arg_values):] on line 736 evaluates to arg_names[-0:] (['element']). zip(..., []) still yields nothing today, though guarding the loop on line 736 with if default_arg_values: would avoid the -0 slice pitfall if strict=True is ever added later.

[xgbtxy]

Added the if default_arg_values: guard in 0dbcf73. This skips the -0 slice when no defaults remain.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.90%. Comparing base (2213e54) to head (52cc6f2).
⚠️ Report is 27 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #40798       +/-   ##
===========================================
+ Coverage        0   79.90%   +79.90%     
===========================================
  Files           0      493      +493     
  Lines           0    83475    +83475     
===========================================
+ Hits            0    66704    +66704     
- Misses          0    16771    +16771     

Flag	Coverage Δ
python	79.90% <100.00%> (?)

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[damccorm]

waiting on author