pytest-subproc 0.1.1

A pytest plugin to run marked tests in a subprocess

pytest-subproc

Run marked pytest test functions in an isolated subprocess to protect the main process from crashes caused by C++ panics, segfaults, or hangs.

Installation

pip install pytest-subproc

Usage

Mark any test with @pytest.mark.subproc to run it in a subprocess:

import pytest

@pytest.mark.subproc
def test_isolated():
    assert True

Parameters

@pytest.mark.subproc(timeout=None, condition=None)

Parameter	Type	Default	Description
timeout	float	None	Timeout in seconds. Kills the subprocess if the test exceeds this limit. Falls back to subproc_default_timeout ini option.
condition	bool or () -> bool	True	When falsy, the test runs in the main process normally. Useful for conditional isolation (e.g., only in CI).

@pytest.mark.subproc(timeout=30)
def test_with_timeout():
    ...

@pytest.mark.subproc(condition=lambda: os.environ.get("CI") == "true")
def test_ci_only():
    ...

@pytest.mark.subproc(timeout=10, condition=False)
def test_never_subprocess():
    ...

Configuration via pyproject.toml / pytest.ini

[tool.pytest.ini_options]
subproc_default_timeout = 30

Or on the command line:

pytest --subprocess-timeout=30

Module-level configuration

Set defaults for all @pytest.mark.subproc tests in a directory tree by calling pytest_subproc methods in a conftest.py:

import pytest_subproc

# Default timeout (lowest priority: marker > CLI > ini > this)
pytest_subproc.config_default_timeout(30)

# Default condition for spawning (lowest priority: marker > this)
pytest_subproc.config_global_enabled(True)
pytest_subproc.config_global_enabled(lambda: os.environ.get("CI") == "true")  # callable

@pytest.mark.timeout interaction

When a test has both @pytest.mark.subproc(timeout=5) and @pytest.mark.timeout(3), the shorter value (3s) is used as the subprocess timeout. This prevents pytest-timeout from killing the main process while the subprocess is still running — our plugin cancels pytest-timeout's timer and enforces the effective timeout on the subprocess itself.

@pytest.mark.subproc(timeout=5)
@pytest.mark.timeout(3)       # ← effective timeout (shorter wins)
def test_obey_the_shorter():
    ...

How It Works

1. Interception — pytest_runtest_protocol (tryfirst) takes over the protocol for subproc-marked tests.

2. Isolated run — The main process spawns a child that runs pytest.main([nodeid, --rootdir, ...]) for that single test. The full lifecycle (conftest loading, fixture resolution, setup, call, teardown) happens inside the subprocess.

3. Timeout & cleanup — The child is created with start_new_session so the entire process group (including test‑spawned children) is killed when the timeout fires.

4. Result — A plugin inside the subprocess captures the outcome and exception; these are pickled to a temp file. The parent re‑raises the exception so xfail, skip, etc. work as expected.

5. Config parity — asyncio_mode, xfail_strict and other ini settings are forwarded to the subprocess via --override-ini.

Comparison with pytest-forked

	pytest-subproc	pytest-forked
Mechanism	Spawns a new Python process (subprocess)	Forks the existing process (os.fork)
Fixtures	Re‑evaluated in the child (full conftest + fixture resolution)	Inherited from parent via copy‑on‑write
Windows	✅ Supported	❌ Not available
Crash isolation	Full — the child has its own PID and memory space; a segfault cannot reach the parent	Partial — forked process shares file descriptors and some kernel state with the parent
Timeout	Built‑in timeout parameter on the marker, with process‑tree termination	Only via external pytest-timeout plugin
Process‑tree cleanup	Kills the entire process group on timeout (os.killpg / taskkill /T)	No automatic child‑process cleanup
Startup cost	Moderate — a new Python interpreter starts and runs pytest.main for one test	Low — fork is (mostly) copy‑on‑write
pytest-xdist	✅ Compatible	✅ Compatible

Feature Compatibility

Feature	Status
Python >= 3.7	✅
Function, session, and module-level fixtures	✅
stdout/stderr captured and shown on failure	✅
pytest.mark.asyncio + asyncio_mode = "auto"	✅
@pytest.mark.parametrize	✅
@pytest.mark.xfail(raises=...)	✅
Custom exception pickling / re-raising	✅
flaky retries	✅
pytest-timeout signal handling	✅
pytest-cov coverage passthrough	✅

Requirements

• Python >= 3.7
• pytest >= 7.0

License

MIT