I created a short script to demonstrate what happens with different identity values. It needs the identity kwarg added in this PR. It shows the get/set value needs work.

Haven't looked at this yet, but the enhancement would be useful.

The ufunc getter for identity here seems to have fewer options than what's defined in the earlier link

The only two that are omited are PyUFunc_None and PyUFunc_ReorderableNone, both of which are returned as None in ufunc.identity. Perhaps there should be two special singleton values here instead, to allow python code to distingish them?

That might make sense. Should I also add a private setter function to convert from input to the internal constants? I think ultimately for some input identity value inp, np.frompyfunc(f, nin, nout, inp).identity == inp (or raise an exception) for all possible values of inp.

At the very least, it'd be nice to get the change supporting kwargs to frompyfunc in

☔ The latest upstream changes (presumably #8861) made this pull request unmergeable. Please resolve the merge conflicts.

Seem to me that we first want to overhaul ufunc.identity to allow any PyObject *, and only then is this worth pursuing. Under the hood, we just convert them to PyObject *s each time we need them anyway.

I've a patch in progress in my tree, but it's unlikely to get many brain cycles in the next month at #8955. Am I right in thinking that appending fields to Py_UfuncObject breaks neither the API nor the ABI?`

#8955 has been merged, PyUFuncObject now has a field that can hold arbitrary PyObject s

@mattharrigan would you like to revive this?

Rebased and solved conflicts. Tweaked the identity argument to be keyword-only.

Test added

Thank Eric and @mattharrigan. The signature addition has been on my/our "Not Done" list for a bit. Should fairly straight forward, but requires a new slightly different inner loop to handle.

@eric-wieser feel free to merge, I think this is good. There are tiny changes in that arguments are now not only positional (which is good). I guess a ping to the mailing list can be good, but since this is a core ufunc feature exposing it to python seems very straight forward. I feel the argument description is a bit difficult, but I cannot figure out how to make it easier. Maybe the link to the ufunc documentation is simply enough.

Hmm, seems I could have just merged it back then. I think it is an obvious enough addition to have the identity kwarg in frompyfunc since it is something we have in the C-API.