can't index string- and unicode- numpy-scalars with an empty tuple

As discovered in #7259 and #7260, string- and unicode- numpy scalars do not support indexing with an empty tuple, unlike other scalars:

>>> numpy.string_("ABC")[()]
TypeError: string indices must be integers, not tuple
>>> numpy.int32(1)[()]
1

This may be a problem because indexing with an empty tuple is a way of converting possible 0-d arrays to scalars:

>>> a = numpy.int64(1)
>>> b = numpy.array(a)
>>> type(a[()]), type(b[()])
(numpy.int64, numpy.int64)

As of writing this it's not 100% clear to me we actually want this. It could be that instead of just writing

return possiblearr[()]

we should encourage people to write something like

if isinstance(possiblearr, ndarray):
    return possiblearr[()]
return possiblearr

But, on the other hand it's probably a good idea for all scalar types to behave similarly. I made a rough outline for a fix in a comment in #7260.

Related: numpy.string_("ABC")[...] fails in the same way

OTOH,

In [5]: numpy.string_("ABC")[1]
Out[5]: 'B'

This manifests itself in np.ma.array(['ab', 'cd']).take(1) due to the patch in #7586

Arguably not a bug - we're never going to have this work on object scalars, so having it fail on string scalars too forces the library writer to use a solution that works for object scalars too.

Closing this older issue in favor of #31435 for the discussion and implementation to stay in one place.

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can't index string- and unicode- numpy-scalars with an empty tuple #7267

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can't index string- and unicode- numpy-scalars with an empty tuple #7267

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eric-wieser commented on Mar 6, 2017

charris commented on Mar 6, 2017

eric-wieser commented on Mar 6, 2017

eric-wieser commented on Jul 3, 2017

InessaPawson commented on Sep 18, 2026

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