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cpython-testing

@gpshead/cpython-skills
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Use this skill when you need to run CPython tests, whether running individual test files, specific test cases, the full test suite, or collecting code coverage. It covers unittest-based testing (not pytest), debugging tests, and test best practices.

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SKILL.md

name cpython-testing
description Use this skill when you need to run CPython tests, whether running individual test files, specific test cases, the full test suite, or collecting code coverage. It covers unittest-based testing (not pytest), debugging tests, and test best practices.

Running CPython Tests

Critical rules:

  1. ALWAYS use sub-agents when running tests
  2. NEVER use pytest - CPython tests are unittest based
  3. Use --match not -k for filtering (this is not pytest!)

Prerequisite: BUILT_PY=build/python or build/python.exe

Running Tests

# Single test module (recommended - proper discovery, parallel execution)
$BUILT_PY -m test test_zipfile -j $(nproc)

# Multiple modules
$BUILT_PY -m test test_csv test_json -j $(nproc)

# Direct execution (quick but may miss test packages)
$BUILT_PY Lib/test/test_csv.py

# Specific test pattern (use --match, NOT -k!)
$BUILT_PY -m test test_zipfile --match "*large*" -j $(nproc)
$BUILT_PY -m test test_csv --match "TestDialect*"
$BUILT_PY -m test test_json --match "TestEncode.test_encode_string"

# Full test suite (ASK FIRST - takes significant time!)
make -C $BUILD_DIR test

# Useful flags: -v (verbose), -f (fail fast), --list-tests (show all), --help

Test packages (directories like test_asyncio/) require load_tests() in __init__.py to work with python -m test.

Code Coverage

# Collect coverage (uses trace mechanism via libregrtest)
$BUILT_PY -m test --coverage test_csv test_json --coveragedir .claude/coverage/ -j $(nproc)

# Reports go to specified coveragedir

Debugging

For interactive debugging (pdb/lldb/gdb) or testing REPL features: Control a tmux session.

Add breakpoint() in test code, then run with -v for verbose output.