pytest-rs 0.0.6

A fast, drop-in compatible pytest runner written in Rust

pytest-rs

pytest-rs is a re-implementation of the popular Python testing framework pytest in Rust, focused on speed: a drop-in compatible runner where startup, collection, fixture orchestration, coverage measurement, and reporting are native code, while test bodies run on embedded CPython.

Note: This project is currently in active development (alpha stage). Many features are still under implementation and subject to change. See docs/DESIGN.md for the architecture and roadmap.

pytest-rs is an independent project, not affiliated with or endorsed by the pytest project.

Installation

Prebuilt wheels are published to PyPI for Linux (x86_64 / aarch64) and macOS (arm64) on CPython 3.13 / 3.14:

uv add --dev pytest-rs    # or: pip install pytest-rs

Then run your existing suite, no changes needed:

pytest-rs                       # whole suite, like `pytest`
pytest-rs tests/test_foo.py     # one file
pytest-rs -n 4                  # parallel workers (pytest-xdist compatible)
pytest-rs --cov=mypkg           # native coverage (pytest-cov compatible)

pytest-rs reads the same configuration pytest does (pytest.ini, pyproject.toml [tool.pytest] / [tool.pytest.ini_options], tox.ini, setup.cfg) and understands the familiar flags (-v, -x, -k, -m, --lf, --tb=..., -p no:NAME, ...). It installs alongside pytest without conflict — the pytest command is untouched.

Requirements

• Linux or macOS (no Windows support yet)
• CPython 3.13+ built with a shared libpython — true for uv-managed Pythons, python.org installers, Homebrew, conda, and distro packages. Plain pyenv builds need PYTHON_CONFIGURE_OPTS="--enable-shared" pyenv install ....

Bundled plugins

The compatibility layers for pytest-asyncio, anyio's pytest plugin, pytest-mock, pytest-cov, pytest-split, pytest-benchmark and pytest-xdist-style -n parallelism are built in — no separate plugin installs (the anyio layer runs tests through the installed anyio library's backends, so anyio itself must be in the environment as usual). Two ways to turn features off:

Per project or per run, like pytest (works with the prebuilt wheel):

[tool.pytest.ini_options]
addopts = "-p no:benchmark -p no:split"

At build time, when installing from source — bundled plugins are Cargo features, all enabled by default:

[tool.uv]
config-settings-package = { pytest-rs = { build-args = "--no-default-features --features asyncio,mock" } }

Third-party plugins (not reimplemented, loaded as-is)

Installed pytest11 entry points load through the pytest API shim — plugins pytest-rs does not reimplement can still work as-is. The supported surface includes fixtures, markers, pytest_addoption (plugin --flags and ini options), config.stash, custom hookspecs (pytest_addhooks), pytest_runtest_protocol/pytest_runtest_call hookwrappers, custom collectors (pytest_collect_file → File/Item), and terminal-reporter replacement (a plugin that unregisters the terminalreporter plugin and registers its own subclass takes over the output — pytest-rs suppresses its native rendering and drives the replacement through the same hooks pluggy would). Verified status:

evidence	plugins
own upstream test suite runs under pytest-rs and gates CI (per-suite pass-rate in the Third-party plugins conformance table below)	pytest-timeout, pytest-randomly, pytest-env, pytest-socket, pytest-snapshot, pytest-ruff, pytest-rerunfailures, pytest-order, pytest-repeat, pytest-instafail, pytest-icdiff, pytest-metadata, pytest-subtests, pytest-mypy, pytest-bdd, pytest-django; anyio's own plugin module also loads this way
functional smoke demo gates CI (conformance/plugin_smoke.py)	Faker, time-machine, requests-mock, inline-snapshot (snapshot assertions + --inline-snapshot flag), pytest-run-parallel (--parallel-threads really runs each test on N threads)
reporter replacement — terminal output byte-diffed against real pytest 9.0.3	pytest-pretty, pytest-sugar (progress bar, instant failures; activates on a tty or --force-sugar)
not reimplemented yet	pytest-html (needs the report data model exposed); syrupy (serializer/extension framework)

A plugin that fails to import (e.g. it reaches into pytest/pluggy internals the shim doesn't provide) warns and is skipped without breaking the run. -p no:NAME and PYTEST_DISABLE_PLUGIN_AUTOLOAD opt out, like pytest.

Performance

Startup, collection, fixture orchestration, coverage measurement, and parallel workers are native Rust code. The main wins:

• --cov runs — coverage is collected via sys.monitoring (Python 3.12+ low-overhead instrumentation) instead of coverage.py's tracing hooks. Typically 1.5–1.9x faster on medium suites, and the gap widens with suite size.
• -n parallel runs — workers are forked (not spawned), so each worker inherits a warm interpreter with all imports already done. Startup per worker drops from seconds to milliseconds.
• Large collections — fixture resolution and parametrize expansion run in Rust; suites with thousands of tests see faster collection.

Benchmarks on open-source projects (macOS arm64, release build, median of 3 warm runs):

suite (tests)	mode	pytest	pytest-rs	speedup
pydantic (12 750)	--cov	3.64 s	1.90 s	1.9x
pydantic (12 750)	--parallel-threads 3 --cov ¹	3.82 s	1.97 s	1.9x
marshmallow (1 119)	--cov	1.65 s	1.07 s	1.5x
marshmallow (1 119)	-n 3 --cov	1.79 s	0.81 s	2.2x
click (1 336)	--cov	3.46 s	2.19 s	1.6x
click (1 336)	-n 3 --cov	3.66 s	2.35 s	1.6x

¹ pydantic's suite runs under pytest-run-parallel (--parallel-threads), so both runners are measured with it. Plain pytest -n 3 (xdist) fails on pydantic with a "different tests collected" error; pytest-rs's fork-based workers avoid this class of issue.

For small, CPU-bound suites without coverage or parallelism the test bodies dominate and both runners perform similarly. Try it on your own suite:

hyperfine -w 1 'pytest -q' 'pytest-rs -q'
hyperfine -w 1 'pytest --cov=mypkg' 'pytest-rs --cov=mypkg'

Known limitations

• unix only (no Windows)
• no --pdb / debugger integration yet
• third-party pytest plugins are loaded via the pytest11 entry point and the pytest API shim; plugins reaching deep into pytest internals may not work (see "Third-party plugins" above for verified examples)

Conformance testing

Compatibility is verified by running the upstream test suites of the libraries pytest-rs reproduces, unchanged, under pytest-rs (conformance/).

Current results (total = passed + failed + errors + skipped; updated automatically by conformance/runner.py, refreshed by CI on every push to main — see conformance/RESULTS.md for per-file detail):

linux (CI-verified)

pytest & plugin ecosystem (the APIs pytest-rs reimplements):

suite	tag	passed	failed	errors	skipped	total	conformant %	files all-pass	files run	files excluded
pytest	9.0.3	2349	429	3	52	2833	84.8%	14	52	63
pytest-asyncio	v1.4.0	268	0	0	0	268	100.0%	30	30	0
pytest-mock	v3.15.1	87	0	0	1	88	100.0%	1	1	0
pytest-cov	v7.1.0	185	21	0	3	209	90.0%	0	1	0
pytest-xdist	v3.8.0	91	6	0	1	98	93.9%	0	1	6
pytest-split	0.9.0	59	0	0	0	59	100.0%	1	1	3
pytest-benchmark	v5.1.0	91	31	0	1	123	74.8%	4	7	6
anyio	4.13.0	3120	0	0	42	3162	100.0%	26	26	0

Third-party plugins (not reimplemented — their own upstream test suites run under pytest-rs, loaded via the pytest11 entry-point shim):

suite	tag	passed	failed	errors	skipped	total	conformant %	files all-pass	files run	files excluded
pytest-timeout	2.4.0	40	0	0	1	41	100.0%	1	1	0
pytest-mypy	v1.0.1	75	3	0	0	78	96.2%	0	1	0
pytest-ruff	v0.5	10	0	0	0	10	100.0%	1	1	0
pytest-subtests	v0.14.2	28	4	0	0	32	87.5%	0	1	0
pytest-metadata	v2.0.4	10	0	0	0	10	100.0%	1	1	0
pytest-snapshot	v0.9.0	101	6	0	0	107	94.4%	0	3	0
pytest-icdiff	0.5	10	2	0	0	12	83.3%	0	1	0
pytest-socket	0.7.0	59	6	0	0	65	90.8%	2	6	0
pytest-order	v1.4.0	115	19	0	0	134	85.8%	7	16	0
pytest-repeat	v0.9.4	16	0	0	0	16	100.0%	1	1	0
pytest-instafail	v0.5.0	63	0	0	0	63	100.0%	1	1	0
pytest-env	1.6.0	67	8	0	0	75	89.3%	2	3	0
pytest-rerunfailures	9.1.1	47	0	0	1	48	100.0%	1	1	0
pytest-randomly	4.1.0	33	4	0	0	37	89.2%	0	1	0
pytest-bdd	8.1.0	112	26	0	1	139	81.3%	27	35	0
pytest-django	v4.9.0	141	68	6	1	216	65.7%	2	13	0

Real-world projects (their suites run unchanged, as drop-in evidence):

suite	tag	passed	failed	errors	skipped	total	conformant %	files all-pass	files run	files excluded
click	8.3.1	1314	0	0	21	1335	100.0%	20	20	0
jinja	3.1.6	909	0	0	0	909	100.0%	22	22	0
marshmallow	4.1.1	1119	0	0	0	1119	100.0%	12	12	3
rich	v14.2.0	855	0	0	25	880	100.0%	60	62	0
httpx	0.28.1	1404	13	0	1	1418	99.1%	26	31	0
httpx2	v2.4.0	1426	0	0	1	1427	100.0%	31	31	0
starlette	0.46.2	902	0	1	0	903	99.9%	27	28	0
attrs	25.3.0	1339	2	0	4	1345	99.9%	20	24	0
more-itertools	v10.7.0	670	0	0	1	671	100.0%	2	2	0
werkzeug	3.1.3	897	1	1	0	899	99.8%	21	25	0
fastapi	0.115.12	2324	8	1	130	2463	99.6%	298	310	0
packaging	25.0	26897	50	110	0	27057	99.4%	11	12	0
pandas	v3.0.3	160750	39	371	25459	186619	99.8%	863	964	0
scikit-learn-1	1.9.0	8299	16	0	6617	14932	99.9%	75	87	0
scikit-learn-2	1.9.0	4915	0	2	1888	6805	100.0%	50	58	0
scikit-learn-3	1.9.0	9240	7	0	2530	11777	99.9%	101	114	0

The suites are included as shallow git submodules under conformance/suites/ at the pinned release tags. Initialize them once after cloning:

git submodule update --init --depth 1

Then run the full conformance harness:

cargo build
uv run --no-project python conformance/runner.py --local   # uses submodules
uv run --no-project python conformance/runner.py           # re-clones from upstream (CI mode)

Project	License	Tag
pytest	MIT	9.0.3
pytest-asyncio	Apache-2.0	v1.4.0
pytest-mock	MIT	v3.15.1
pytest-cov	MIT	v7.1.0
pytest-xdist	MIT	v3.8.0
pytest-split	MIT	0.9.0
pytest-benchmark	BSD-2-Clause	v5.1.0

pytest-rs reimplements the public APIs of these projects, plus anyio's pytest plugin (MIT). Parts of the bundled Python shims are ports of upstream code; see THIRD-PARTY-NOTICES.md. Credit for the API design and the test suites belongs to their respective authors.

License

This project is licensed under the MIT License. See the LICENSE file for more details.