Subsetting w/ empty list when X is dense throws error

[ivirshup]

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• I have checked that this issue has not already been reported.
• I have confirmed this bug exists on the latest version of anndata.
• (optional) I have confirmed this bug exists on the master branch of anndata.

Report

Code:

import numpy as np, anndata as ad

ad.AnnData(np.ones((3, 3)))[:, []].X

Traceback:

---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[4], line 1
----> 1 ad.AnnData(np.ones((3, 3)))[:, []].X

File ~/github/anndata/anndata/_core/anndata.py:656, in AnnData.X(self)
    653     X = None
    654 elif self.is_view:
    655     X = as_view(
--> 656         _subset(self._adata_ref.X, (self._oidx, self._vidx)),
    657         ElementRef(self, "X"),
    658     )
    659 else:
    660     X = self._X

File /usr/local/Cellar/python@3.9/3.9.18/Frameworks/Python.framework/Versions/3.9/lib/python3.9/functools.py:888, in singledispatch.<locals>.wrapper(*args, **kw)
    884 if not args:
    885     raise TypeError(f'{funcname} requires at least '
    886                     '1 positional argument')
--> 888 return dispatch(args[0].__class__)(*args, **kw)

File ~/github/anndata/anndata/_core/index.py:144, in _subset(a, subset_idx)
    142 if all(isinstance(x, cabc.Iterable) for x in subset_idx):
    143     subset_idx = np.ix_(*subset_idx)
--> 144 return a[subset_idx]

IndexError: arrays used as indices must be of integer (or boolean) type

Works when X is sparse. Probably not a super common use case.

Versions

-----
IPython             8.14.0
anndata             0.10.0rc2.dev2+gfa8149d9.d20230927
numpy               1.24.4
session_info        1.0.0
-----
asciitree           NA
asttokens           NA
awkward             2.3.1
awkward_cpp         NA
backcall            0.2.0
cloudpickle         2.2.1
colorama            0.4.6
cython_runtime      NA
cytoolz             0.10.1
dask                2023.9.1
dateutil            2.8.2
decorator           5.1.0
entrypoints         0.3
exceptiongroup      1.0.4
executing           0.8.2
fasteners           0.17.3
google              NA
h5py                3.9.0
importlib_metadata  NA
jedi                0.18.1
jinja2              3.1.1
llvmlite            0.40.1
markupsafe          2.1.1
msgpack             1.0.3
natsort             8.3.0
numba               0.57.1
numcodecs           0.10.2
numexpr             2.8.1
packaging           23.1
pandas              2.1.0
parso               0.8.2
pexpect             4.8.0
pickleshare         0.7.5
prompt_toolkit      3.0.38
psutil              5.9.0
ptyprocess          0.7.0
pure_eval           0.2.1
pyarrow             12.0.1
pygments            2.16.1
pytz                2022.7.1
regex               2.5.112
ruamel              NA
scipy               1.11.2
setuptools_scm      NA
sitecustomize       NA
six                 1.16.0
sparse              0.14.0
sphinxcontrib       NA
stack_data          0.1.4
tblib               1.7.0
tlz                 0.10.1
toolz               0.12.0
traitlets           5.6.0
typing_extensions   NA
wcwidth             0.2.5
xxhash              NA
yaml                5.4.1
zappy               NA
zarr                2.16.1
zipp                NA
zoneinfo            NA
-----
Python 3.9.18 (main, Aug 24 2023, 21:32:48) [Clang 14.0.3 (clang-1403.0.22.14.1)]
macOS-13.4.1-x86_64-i386-64bit
-----
Session information updated at 2023-09-27 12:34