[Amar3tto]

Fixes #40660, fixes #40661

Successful runs:
https://github.com/apache/beam/actions/runs/39405250368
https://github.com/apache/beam/actions/runs/39405225694

[github-actions]

📋 Review Summary

This PR fixes type checking errors in the SpannerIO Python performance tests (spannerio_read_perf_test.py and spannerio_write_perf_test.py) by explicitly specifying .with_output_types(_Mutator) on the FlatMap(make_insert_mutations) transform.

🔍 General Feedback

• The changes are clean, minimal, and directly resolve the type inference mismatch between FlatMap and _MakeMutationGroupsFn (which expects Union[MutationGroup, _Mutator]).
• Both read and write performance test workflows have been verified with passing runs.

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[damccorm]

Do we know what change caused this to start failing? Did we make a breaking change to something?

[Amar3tto]

I found a possible trigger: #40413, merged on October 8, switched Python load/performance tests from 3.10 to 3.11. This may have exposed the existing FlatMap type inference issue. I haven't confirmed it yet, but I didn't find any changes to the SpannerIO API or type checking itself.

[damccorm]

Ok - could we try to run before/after that change and see if it shows the issue? If yes, then I am fine with the change. If no, we should really understand why we need this first

[Amar3tto]

Confirmed - Python 3.10 works https://github.com/apache/beam/actions/runs/39619250068

[github-actions]

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