8000 BUG: Attempting to use arrays as slices gives an inappropriate error message · Issue #28486 · numpy/numpy · GitHub
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BUG: Attempting to use arrays as slices gives an inappropriate error message #28486

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Orisphera opened this issue Mar 12, 2025 · 4 comments
Closed

BUG: Attempting to use arrays as slices gives an inappropriate error message #28486

Orisphera opened this issue Mar 12, 2025 · 4 comments
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00 - Bug

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@Orisphera
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Describe the issue:

When attempting to use arrays as slices, NumPy gives a message that doesn't accurately describe the actual problem.

Reproduce the code example:

import numpy as np

a = np.zeros(10)
i = np.arange(2, 4)
a[i::i] = 1

Error message:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[8], line 4
      2 b = np.ones(s, dtype=bool)
      3 p = np.arange(s)[a]
----> 4 b[p ** 2::p] = False

TypeError: only integer scalar arrays can be converted to a scalar index

Python and NumPy Versions:

1.26.4
3.12.8 (main, Dec 3 2024, 18:42:41) [GCC 13.3.0]

Runtime Environment:

[{'numpy_version': '1.26.4',
'python': '3.12.8 (main, Dec 3 2024, 18:42:41) [GCC 13.3.0]',
'uname': uname_result(system='Linux', node='nixos', release='6.6.30-rt30', version='#1-NixOS SMP PREEMPT_RT Thu May 2 14:32:50 UTC 2024', machine='x86_64')},
{'simd_extensions': {'baseline': ['SSE', 'SSE2', 'SSE3'],
'found': ['SSSE3',
'SSE41',
'POPCNT',
'SSE42',
'AVX',
'F16C',
'FMA3',
'AVX2'],
'not_found': ['AVX512F',
'AVX512CD',
'AVX512_KNL',
'AVX512_KNM',
'AVX512_SKX',
'AVX512_CLX',
'AVX512_CNL',
'AVX512_ICL',
'AVX512_SPR']}},
{'filepath': '/nix/store/mhd0rk497xm0xnip7262xdw9bylvzh99-gcc-13.3.0-lib/lib/libgomp.so.1.0.0',
'internal_api': 'openmp',
'num_threads': 2,
'prefix': 'libgomp',
'user_api': 'openmp',
'version': None}]

Context for the issue:

I found this issue while playing around with NumPy, and it really confused me. I think people may get equally confused when doing some actual work.
@ngoldbaum
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What phrasing would have helped you more? IMO the current message describes what the issue is. Keep in mind that some arrays (those with shape ()) can be used as a scalar index:

In [3]: a = np.zeros(10)

In [4]: i = np.array(2)

In [5]: a[i::i] = 1

In [6]: a
Out[6]: array([0., 0., 1., 0., 1., 0., 1., 0., 1., 0.])

In [7]: i.shape
Out[7]: ()

@Orisphera
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I misread the error message

@Orisphera Orisphera closed this as not planned Won't fix, can't repro, duplicate, stale Mar 12, 2025
@Orisphera
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Still, I think it would be nice to mention that NumPy can't change multiple slices at once

@ngoldbaum
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It is possible to do what (I think?) you were trying to do but it requires using advanced indexing.

There's certainly room to make the error message more clearly communicate what the fundamental issue is: when you use the start:stop:step style indexing, NumPy treats the indices as scalars.

Feel free to reopen the issue btw, there's always room to make errors clearer. I was just trying to point out that there are some subtleties.

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