coverage-instrumentation-pattern

What problem does it solve?

Prevents misleading Python test coverage results when using pytest in Python 3.12+ with async frameworks like FastAPI and httpx, by selecting the correct coverage backend to avoid false missing lines.

Core Features & Use Cases

• Backend comparison (ctrace vs sysmon): Identifies when the default coverage backend under-measures async call stacks and switches to PEP-669 sys.monitoring.
• Fast activation via config or env: Supports enabling sysmon using the COVERAGE_CORE environment variable, .coveragerc, or pyproject.toml.
• CI gate re-calibration guidance: Recommends adjusting cov-fail-under thresholds after changing the backend so coverage checks remain meaningful.

Use case: You add or modify tests, yet gateway/admin logic still shows unexpectedly low coverage because async routes and ASGI middleware aren’t fully captured by ctrace.

Quick Start

Run pytest with the sysmon backend by setting COVERAGE_CORE to sysmon, for example for your source directory: COVERAGE_CORE=sysmon pytest --cov=src/.