Yeah it is a bit clunky right now. In array-api-tests we cast 0d array floats to builtin floats, before using math.copysign(1, your_builtin_float) == 1 to see whether its positively signed or not. Maybe it's not too big of a deal to add a function for it (or possibly modifying sign), don't have a sense for that.

There is a proposal for copysign at #593.

which brings up a question on how implementors can distinguish between -0.0 and +0.0 in their test suite.

For an implementer to test something, it isn't required to use only functions in the standard. This is checking the value of a bit, which can be done with a private function even.

Not saying that it's not a good idea to have this function - but especially given that JAX doesn't have it, it'd be nice to have a stronger reason.

I checked the SciPy code base. There's a lot of use of the C/C++ versions of signbit in compiled code, but only a couple in Python code:

scipy/special/tests/test_wrightomega.py
55:        assert_(np.signbit(res.imag) == np.bool_(False))

scipy/special/tests/test_trig.py
58:    assert np.signbit(y)
62:    assert not np.signbit(y)
66:    assert not np.signbit(y)

scipy/linalg/_matfuncs_expm.pyx.in
561:            S[:, t:] = np.signbit(Y)

Scikit-learn doesn't have any usage of signbit, and Pandas has four instances:

pandas/tests/indexes/test_numpy_compat.py
105:    "func", [np.isfinite, np.isinf, np.isnan, np.signbit], ids=lambda x: x.__name__
128:        is_complex_dtype(index) and func is np.signbit

pandas/tests/indexes/multi/test_analytics.py
253:    [np.isfinite, np.isinf, np.isnan, np.signbit],

pandas/tests/arithmetic/test_numeric.py
45:    if np.signbit(np.array(zero)).any():
48:        assert np.signbit(np.array(zero)).all()

pandas/core/ops/missing.py
111:        zneg_mask = zmask & np.signbit(y)

JAX does have signbit: https://jax.readthedocs.io/en/latest/_autosummary/jax.numpy.signbit.html

Okay, that is two votes for adding signbit and all libraries have it, so there are no blockers. The signature is as simple as it gets:

signbit(x: array, /) -> array

Let's add it?

I can create a PR later this week.

PR is up for review: gh-705