feat: collect coverage data from Python subprocesses#1055
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When coverage is enabled, propagate the configuration to subprocesses so that pytest-xdist workers, subprocess.Popen calls, and other child processes sharing the same venv automatically measure coverage. At setup time, write a .pth file into the venv's site-packages so every subprocess calls coverage.process_startup() at interpreter init, and set COVERAGE_PROCESS_CONFIG with the serialized config. At teardown, save() + combine() merges per-process data files before generating the LCOV report. Requires coverage >= 7.10.3 for CoverageConfig.serialize(). Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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Summary
When coverage is enabled (
bazel coverage), propagate the configuration to Python subprocesses so that pytest-xdist workers,subprocess.Popencalls, and other child processes sharing the same venv automatically measure coverage.Problem:
pytest_main.pyonly collects coverage from the in-process test code. Code executing in subprocesses (e.g. pytest-xdist workers, or tests that spawn Python subprocesses) is invisible to coverage.py, which can causeNoDataErrorcrashes or incomplete coverage reports.Fix (~15 lines in
pytest_main.py):.pthfile to the venv's site-packages so every subprocess callscoverage.process_startup()at interpreter init. SetCOVERAGE_PROCESS_CONFIGwith the serialized config. Usedata_suffix=Trueso per-process data files don't collide.save()+combine()merges all per-process data files before generating the LCOV report.Also bumps
coverageminimum to>=7.10.3forCoverageConfig.serialize().Scope
This covers subprocesses that share the test's venv (pytest-xdist workers, same-venv subprocess calls). Cross-binary coverage (subprocess of a separate Bazel binary with its own venv) remains unsupported — that would require changes to binary build rules.
Test plan
coverage_lcov_shape_testpasses (no regression)subprocess_coverage_lcov_test— callsfoo.subtract()in a subprocess, asserts the LCOV containsDA:5,<nonzero>proving subprocess coverage was captured//examples/pytest/...tests pass (10/10)🤖 Generated with Claude Code