rstest

Run your existing pytest suite in parallel, unchanged. Same fixtures, same plugins, same outcomes — just faster. Rust-orchestrated, parallel by default, with built-in suite diagnostics (--doctor) that tell you where your test time actually goes.

aiohttp, 4,469 tests:   pytest 197s  →  rstest 68s warm (151s cold)

📚 Full documentation → python-rstest.readthedocs.io

Quick start

In any pytest project:

pip install rstest      # or: uv pip install rstest
rstest

Also works with your tool of choice:

uv add rstest           # uv projects
poetry add rstest       # Poetry
pdm add rstest          # PDM
pipx install rstest     # isolated CLI install

No config. No test changes. If pytest runs it, rstest runs it — in parallel (-n auto), out of the box.

Tests that can't run in parallel → @pytest.mark.serial (run exclusively, after the parallel phase).
Order-dependent suites → --dist loadfile.
Want byte-exact pytest semantics → rstest -n 0 (single pytest session).

Will rstest speed up your suite?

The wins come from suite shape, not magic. Quick self-check:

Your suite	What to expect
Wait-bound (IO, sleeps, network, timeouts)	Biggest win — test-granular dispatch splits the slow files xdist pins to one worker.
CPU-bound, already splits well under xdist	Parity, not a win — gain up to core count, same as xdist (pandas: 63s vs 61s).
Gated by one long test	No win beyond that test — no worker count beats the long pole. `--doctor` names it.
Small (< ~10s serial)	Little wall-time change — value is `--watch`, `--changed`, `--doctor`, not raw speed.
Many tiny per-service suites	Speedup is per-suite; the aggregate CI win depends on your largest suites.

Two more truths worth knowing before you benchmark:

Warm cache matters. Duration-aware scheduling needs one run of timing data. First run is cold; the win arrives on run two. In ephemeral CI, persist .rstest_cache or expect cold-run timing.
Adopting rstest adopts pytest 9. rstest runs a vendored pytest 9.1.1 core. A suite that's warning-clean on recent pytest 8.x is almost always already pytest-9-clean; if not, clear deprecations first (the same upgrade you'd owe pytest anyway). rstest -n 0 surfaces them.

Fastest way to find out for real: rstest --try runs your suite under plain pytest and under rstest -n auto, then reports whether outcomes match and how much faster rstest was — no migration, no config.

Benchmarks

Real open-source suites, end-to-end, with per-test outcome diffing against the pytest baseline — 100% parity on the measured run (a few tests flake under plain pytest itself; those are catalogued in the docs).

Suite	Tests	pytest	xdist (`-n 8`)	rstest
aiohttp	4,469	197s	160s	68s warm · 151s cold
pandas	193,627	182s	61s	63s (parity, not a win)
django-allauth	2,050	22s	8s	8s (`-n 4`)
rich	981	3.4s	2.8s	2.5s (`-n 4`)

Apple Silicon, CPython 3.13, pytest-xdist 3.8.

Monorepo (langchain-ai/langgraph, 6 libs/* packages, 4,284 tests, each with its own pytest config — a single pytest can't run from the root at all):

	wall	parity
pytest — 6 serial invocations	880.4s	baseline
rstest at the root, cold	245.7s (3.6×)	100% (measured)
rstest at the root, warm	121–133s (6.6–7.3×)	projected

The cold run is measured. The warm figure is a projection, not a recorded run — the cold run's per-package duration caches predict where the planner lands once warm, so it carries no parity number. Discount it until you measure your own.

Full methodology: benchmarks.

Why rstest

	pytest	pytest-xdist	rstest
Runs your suite unchanged	✅	mostly	✅
Parallel by default	❌	⚙️ opt-in	✅
Duration-aware scheduling	❌	❌	✅
Crashed workers respawn mid-run	❌	❌	✅
Suite diagnostics	❌	❌	✅ `--doctor`
Watch mode	plugin	❌	✅ built-in

Drop-in. Forwards the pytest flag surface; runs conftest, fixtures, parametrize, marks, and pytest plugins (pytest-django, pytest-asyncio, hypothesis, …) through a vendored pytest core.
Parallel by design. Test-granular work distribution with duration-aware scheduling; @pytest.mark.serial and --dist loadfile safety rails; crashed workers respawn without losing your run.
rstest --doctor. Wait-bound tests, parallel-floor analysis, fixture hotspots, slowest files.
rstest --watch. Instant reruns on save; changed test files rerun alone, source changes rerun only the tests the import graph says are affected.

Compatibility contract

At -n 0, outcomes are byte-exact — one vendored-pytest session; any difference at -n 0 is a bug. In parallel modes, outcomes are preserved for parallel-safe tests; tests with hidden time/ordering/shared-state assumptions can flake under high concurrency — exactly as under pytest-xdist. rstest --doctor and lower -n values help find and contain them; @pytest.mark.serial is the escape hatch.

Vendored pytest 9. rstest runs a vendored pytest 9.1.1 core, so adopting rstest adopts pytest 9's behavior regardless of the pytest version installed. The 8→9 gap is a cleanup major (removes already-deprecated APIs); a suite warning-clean on recent pytest 8.x is almost always pytest-9-clean. If it isn't, clear the deprecations first — the same upgrade you'd owe pytest anyway. There is one vendored core, tracked forward; no older-core build.

Silent-at--n ≥ 2 plugins. A few plugins that need a single master process are no-ops in parallel: --html (pytest-html) writes nothing (no crash) — generate reports at -n 0/-n 1. Terminal-UI plugins (pytest-sugar and friends) don't paint; data-level behavior is unaffected. Full list: Known gaps.

`rstest --doctor`

Runs your suite, then answers where does the time actually go? — from data the runner already owns (per-test wall/CPU time, per-fixture setup):

================== rstest doctor ==================
4442 tests, 185.8s test time (wall 67.7s, 8 workers)

WAIT-BOUND: 95% of test time (176.5s) is waiting, not computing (sleeps / IO / timeouts).
    54.20s waiting of   54.25s  tests/test_proxy_functional.py::test_proxy_https_multi_conn_limit
  ... and 33 more

PARALLEL FLOOR: the longest test (54.2s) exceeds the ideal per-worker share (23.2s at -n 8);
no worker count can finish faster than its longest test.

FIXTURE HOTSPOTS (setup time across all workers):
     0.79s   4442x  scope=function blockbuster

SLOWEST FILES:
   150.46s (81.0%)  tests/test_proxy_functional.py
===================================================

That's aiohttp's real suite — one file is 81% of total test time, almost all of it waiting on 10-second proxy timeouts. More →

Docs

License

Licensed under either of

Apache License, Version 2.0 (LICENSE-APACHE or http://www.apache.org/licenses/LICENSE-2.0)
MIT license (LICENSE-MIT or http://opensource.org/licenses/MIT)

at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Development Status
- 3 - Alpha
Programming Language
- Python :: 3
- Rust
Topic
- Software Development :: Testing

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