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BUG: don't assume strides are a multiple of itemsize in stringdtype casts - #32273

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ngoldbaum merged 2 commits into
numpy:mainfrom
ngoldbaum:fix-cast-strides-division
Aug 13, 2026
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BUG: don't assume strides are a multiple of itemsize in stringdtype casts#32273
ngoldbaum merged 2 commits into
numpy:mainfrom
ngoldbaum:fix-cast-strides-division

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@ngoldbaum

@ngoldbaum ngoldbaum commented Aug 12, 2026

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PR summary

See the test I added. On NumPy 2.5.2 you can get a cast to generate garbage by casting from void to StringDType:

>>> rec = np.zeros(3, dtype=[("c", "c16"), ("f", "f8")])
... rec["c"] = [1 + 1j, 2 + 2j, 3 + 3j]
... rec["f"] = [10.0, 20.0, 30.0]
...
>>> rec["c"].astype("T")
array(['(1+1j)', '(10+2j)', '(2+20j)'], dtype=StringDType())
>>> rec["c"]
array([1.+1.j, 2.+2.j, 3.+3.j])
>>> np.__version__
'2.5.2'

AI Disclosure

An AI model spotted this bug and helped me with the test case.

@ngoldbaum ngoldbaum added 00 - Bug 09 - Backport-Candidate PRs tagged should be backported labels Aug 12, 2026
@MaanasArora

MaanasArora commented Aug 13, 2026

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Thanks, should we not test for datetime/timedelta as well? Otherwise the fix looks good to me! (unicode doesn't seem to be impacted, so makes sense to skip it.)

@ngoldbaum

ngoldbaum commented Aug 13, 2026

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Thanks for giving this a look @MaanasArora. I added the tests you asked for and I expect them to be non-controversial so I'll just hit the merge button.

@ngoldbaum
ngoldbaum merged commit dd18992 into numpy:main Aug 13, 2026
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@charris charris mentioned this pull request Aug 14, 2026
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@charris charris removed the 09 - Backport-Candidate PRs tagged should be backported label Aug 14, 2026
charris pushed a commit that referenced this pull request Aug 14, 2026
@ngoldbaum @charris
BUG: don't assume strides are a multiple of itemsize in stringdtype c…
ad99d74 
…asts (#32273)
charris added a commit that referenced this pull request Aug 14, 2026
@charris
Merge pull request #32292 from charris/backport-32273
cf919f2 
BUG: don't assume strides are a multiple of itemsize in stringdtype casts (#32273)
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@ngoldbaum @MaanasArora @charris