BUG: `lazy_xp_function` is incompatible with `pytest-run-parallel`

[crusaderky]

In both array-api-extra and scipy, the xp fixture depends on monkeypatch, as required by lazy_xp_function.
This in turn causes pytest-run-parallel to automatically disable parallel testing for all tests that use xp, for all backends, including when SCIPY_ARRAY_API is not set.

Desired behaviour

Currently, there is actually no need to disable parallelisation, due to a missing feature: Quansight-Labs/pytest-run-parallel#14
Assuming that at some point soon the linked issue is resolved, thread parallelization should exclusively be disabled for those backends that actually require patching, which means JAX and Dask.

[crusaderky]

CC @rgommers
Iif you agree I'm going to tackle this as a matter of high priority.

[crusaderky]

I'm inclined to change from this

@pytest.fixture(params=[
    numpy,
    array_api_strict,
    torch,
    cupy,
    dask.array,
    jax.numpy,
])
def xp(request, monkeypatch):
    xpx.testing.patch_lazy_xp_functions(xp, request, monkeypatch)
    return request.param

def patch_lazy_xp_functions(xp, request, monkeypatch):
    if is_jax_namespace(xp) or is_dask_namespace(xp):
        monkeypatch.setattr(...)

to this:

@pytest.fixture(params=[
    numpy,
    array_api_strict,
    torch,
    cupy,
    pytest.param(dask.array, marks=[pytest.mark.thread_unsafe]),
    pytest.param(jax.numpy, marks=[pytest.mark.thread_unsafe]),
])
def xp(request):
    with xpx.testing.patch_lazy_xp_functions(xp, request):
        yield request.param

@contextmanager
def patch_lazy_xp_functions(xp, request):
    if is_jax_namespace(xp) or is_dask_namespace(xp):
        with unittest.patch(...):
            yield
    else:
        yield

[rgommers]

Yes, I agree that this needs improving, would be great if you could tackle it @crusaderky.

It's tricky to judge the unittest.patch change, but if it works then sure why not. The other option would be to define xp as a no-op fixture unless the backend requested isn't none or ['numpy'].

[crusaderky]

The other option would be to define xp as a no-op fixture unless the backend requested isn't none or ['numpy'].

The problem with that is that I can't conditionally depend on monkeypatch just for selected params.