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ENH: infer nin/nout for frompyfunc#30670

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danielzgtg wants to merge 3 commits intonumpy:mainfrom
danielzgtg:feat/frompyfuncInferCoargcount
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ENH: infer nin/nout for frompyfunc#30670
danielzgtg wants to merge 3 commits intonumpy:mainfrom
danielzgtg:feat/frompyfuncInferCoargcount

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@danielzgtg danielzgtg commented Jan 19, 2026

This eliminates redundantly typing nin/nout especially when obvious with lambda.

Before

home@daniel-desktop3:~$ python3
Python 3.13.7 (main, Nov 24 2025, 20:51:28) [GCC 15.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> np.frompyfunc(lambda x: x, 1, 1).nin
1
>>> np.frompyfunc(lambda x: x, 1, 1).nout
1
>>> np.frompyfunc(lambda x: x + y, 2, 1).nin
2
>>> np.frompyfunc(lambda x: x + y, 2, 1).nout
1
>>> np.frompyfunc(lambda x: x)
Traceback (most recent call last):
  File "<python-input-1>", line 1, in <module>
    np.frompyfunc(lambda x: x)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^
TypeError: frompyfunc() missing required argument 'nin' (pos 2)
>>> np.frompyfunc(lambda x, y: x + y)
Traceback (most recent call last):
  File "<python-input-2>", line 1, in <module>
    np.frompyfunc(lambda x, y: x + y)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
TypeError: frompyfunc() missing required argument 'nin' (pos 2)
>>>

After

(venv) home@daniel-desktop3:~/CLionProjects/numpy$ spin python
Invoking `build` prior to invoking Python:
$ /home/home/CLionProjects/numpy/venv/bin/python3 vendored-meson/meson/meson.py compile -C build
INFO: autodetecting backend as ninja
INFO: calculating backend command to run: /home/home/CLionProjects/numpy/venv/bin/ninja -C /home/home/CLionProjects/numpy/build
ninja: Entering directory `/home/home/CLionProjects/numpy/build'
[1/1] Generating numpy/generate-version with a custom command
Saving version to numpy/version.py
$ /home/home/CLionProjects/numpy/venv/bin/python3 vendored-meson/meson/meson.py install --only-changed -C build --destdir ../build-install
$ export PYTHONPATH="/home/home/CLionProjects/numpy/build-install/usr/lib/python3/dist-packages"
🐍 Launching Python with PYTHONPATH="/home/home/CLionProjects/numpy/build-install/usr/lib/python3/dist-packages"
$ /home/home/CLionProjects/numpy/venv/bin/python3 -P
Python 3.13.7 (main, Nov 24 2025, 20:51:28) [GCC 15.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import numpy as np
>>> np.frompyfunc(lambda x: x).nin
1
>>> np.frompyfunc(lambda x: x).nout
1
>>> np.frompyfunc(lambda x, y: x + y).nin
2
>>> np.frompyfunc(lambda x, y: x + y).nout
1
>>>

jorenham
jorenham requested changes Jan 19, 2026
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Thanks; sounds like a good improvement to me (assuming that it works).

This will need runtime test, e.g. to verify that non-function callables like

class F:
    def __call__(self, x):
        return x

assert np.frompyfunc(F(), 1, 1)(42) == 42

still work.

@danielzgtg danielzgtg force-pushed the feat/frompyfuncInferCoargcount branch 2 times, most recently from c1cea0d to 003b2cc Compare January 19, 2026 13:31
@jorenham
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jorenham commented Jan 19, 2026

The docstring signatures also need to be updated

numpy/numpy/_core/_add_newdocs.py

Lines 4886 to 4891 in 4e2d3f0

add_newdoc('numpy._core.umath', 'frompyfunc',
"""
frompyfunc(func, /, nin, nout, **kwargs)
--
frompyfunc(func, /, nin, nout, *[, identity])

That'll (magically) also fix the stubtest errors :)

@danielzgtg danielzgtg force-pushed the feat/frompyfuncInferCoargcount branch 2 times, most recently from d9cd8c5 to cab0c24 Compare January 19, 2026 14:27
jorenham
jorenham reviewed Jan 19, 2026
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jorenham
jorenham approved these changes Jan 19, 2026
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Ok, all good from me on the typing side. It might be good for another maintainer to look at this before merging though.

seberg
seberg reviewed Jan 20, 2026
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@danielzgtg danielzgtg force-pushed the feat/frompyfuncInferCoargcount branch 4 times, most recently from 2cca835 to bf204db Compare January 20, 2026 12:58
@danielzgtg danielzgtg requested a review from seberg January 21, 2026 15:29
@danielzgtg danielzgtg force-pushed the feat/frompyfuncInferCoargcount branch 2 times, most recently from 09338f3 to d8b9292 Compare January 22, 2026 11:49
@danielzgtg danielzgtg mentioned this pull request Jan 22, 2026
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@danielzgtg danielzgtg force-pushed the feat/frompyfuncInferCoargcount branch 2 times, most recently from 6cd37f6 to 4494eb4 Compare January 22, 2026 14:34
@danielzgtg @jorenham
TST: Verify frompyfunc(nonfunction callable)
2f9de76 
Co-authored-by: Joren Hammudoglu <jhammudoglu@gmail.com>
@danielzgtg danielzgtg force-pushed the feat/frompyfuncInferCoargcount branch from 4494eb4 to 2f9de76 Compare January 22, 2026 15:54
@mattip
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mattip commented Jan 26, 2026

Seems to be a net positive. Do we have any idea how this might break current usage? Trying to prevent a regression like #24530

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@jorenham jorenham jorenham approved these changes

@seberg seberg Awaiting requested review from seberg

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