Open seibert opened 5 years ago
This cross
behavior is clearly a problem, for Numba, for @WarrenWeckesser's ufunc work, we came across it in the array API standardization work, and PyTorch/JAX et al will also have issues for the same reasons. It's probably worth removing the length-2 behavior from np.cross
.
Separately, I think there's a bigger thing to learn here. This is probably one of quite a few pain points for Numba. Numba has a hard job keeping up with new NumPy releases (I've been seeing a few complaints/concerns about NumPy 1.22.x not yet being supported), and it's in the interest of both projects to make it easier for Numba to implement support for NumPy functionality.
We could for example make a roadmap item to prioritize and work on pain points in NumPy for Numba. The array API standard work helps a bit here (given a design principle is that everything should be JIT-compilable), however that only covers ~140 or so functions, which is ~10% of NumPy's API surface.
@seibert do you happen to have a wish list? Or is this scattered over many Numba and NumPy issues plus hacks in your code base?
@seibert do you happen to have a wish list? Or is this scattered over many Numba and NumPy issues plus hacks in your code base?
xref https://github.com/numba/numba/issues/8008 for a detailed write-up from the Numba team. Also see the related mailing list message I just sent to make the next step here: https://mail.python.org/archives/list/numpy-discussion@python.org/thread/QL6BTNYZC3UXBUAWMCMO7KZJTDWBBPCO/
I noticed that the #26640 was closed despite having a complete implementation, because there is a one-line equivalent. But I think this is a sufficiently common functionality to still warrant it's own function. The biggest benefit of a standalone function is that it's much easier to remember than the expansion (I can remember the right hand rule but I can never remember the order of the terms). There's also the matter of custom axes which is a simple keyword argument in the function without requiring complex indexing.
A Numba contributor has been working on adding support for
np.cross
to Numba (numba/numba#4128), and this raised the issue thatnp.cross
has an unusual type signature. It feels like two related, but distinct functions have been glued together:A gufunc-like function that returns 3D vector cross products of 3D vector inputs, with the convenience feature to assume a zero z-component if the final dimension of (only) one of the input arrays has length 2. The number of dimensions of the output is equal to the number of dimensions on the inputs.
A different gufunc-like function that returns the z component of the 3D vector cross product of 2D vector inputs. This is only selected if both inputs have length two in their last dimension. The number of dimensions on the output is one less than the number of dimensions of the inputs.
Aside from making the documentation of this function confusing (it took me several tries to understand these two modes, assuming I'm not still confused), it also makes it not possible to write a Numba type signature for this function because the number of returned dimensions depends on the exact length of the last dimension on each input. (Numba considers ndim, layout, and dtype part of the array type for purposes of code generation, but not the actual contents of shape.)
One could argue this is a Numba problem rather than a NumPy problem, but we suspect that attempts to construct a typing hinting system (like https://github.com/numpy/numpy-stubs) for NumPy functions that describes the relationship between input and output dimensions will also trip over
np.cross
. Our workaround in Numba will be only support mode #1 above, and create a separatenumba.cross2d
function that does mode #2, and raise compilations to direct users to the right one.For similar reasons, adding a
np.cross2d
to NumPy might be a useful way to evolve toward an easier to understandnp.cross
and also make type hinting possible, although backward compatibility would require supporting both modes innp.cross
for some time.It is also totally fair to mark this as WONTFIX because this ship has already sailed. :)