8000 Inconsistent indexing result depending on argument type · Issue #6564 · numpy/numpy · GitHub
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llvilanova opened this issue Oct 26, 2015 · 5 comments
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Inconsistent indexing result depending on argument type #6564

llvilanova opened this issue Oct 26, 2015 · 5 comments

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@llvilanova
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The following results seem inconsistent to me (numpy 1.9.2):

>>> a = np.arange(8).reshape((2,2,2))
>>> a[ [[0], [0]] ]
array([[0, 1]])
>>> a[ np.array([[0], [0]]) ]
array([[[[0, 1],
         [2, 3]]],
       [[[0, 1],
         [2, 3]]]])

I've briefly looked at the indexing manual (http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html) and nothing seems to account for this.

Thanks,
Lluis
@njsmith
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njsmith commented Oct 27, 2015

Well, that looks weird indeed. Normally I'd ignore this and wait for @seberg to respond, but I think he's on vacation :-)

I think what's going on is... well, first, background: Python has a weird calling convention for indexing, where if you write [obj] without any commas then __getitem__ gets passed obj directly, but if you write [obj1, obj2] with a comma then __getitem__ gets passed a tuple (obj1, obj2). A consequence of this is that it's impossible to tell the difference between getting passed a single tuple [(obj1, obj2)] versus getting passed two arguments [obj1, obj2]. This means that __getitem__ has to have some special case code to figure out when to unpack tuples and stuff. So far so good. I think what's going on here is that numpy is seeing a[ [[0], [0]] ] and incorrectly treating the outermost list like it were a tuple, i.e., it's treated like it were a[ ([0], [0]) ], which is equivalent to a[ [0], [0] ].

This looks like a bug to me, but it might be one of those bugs that we can't fix without breaking a bunch of code, or where there's some complicated deprecation required or something... probably we should just wait until @seberg gets back because he'll probably know off the top of his head which type of weird case this one is :-)

@seberg
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seberg commented Oct 27, 2015

exactly, i have an opwn PR to deprecate it, might need some work though. the docs do have a warning somewhere, but the excat rules are a bit funny and nowherr explained

greetings from esfahan, iran :).
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@seberg
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seberg commented Nov 10, 2015

There is a warning here http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html#advanced-indexing which basically hints at the problem.

IIRC the exact rule is: A sequence is treated like a tuple, if:

  1. It has no more then 32 entries (maximum number of dimensions)
  2. It contains at least one of a sequence, None, Ellipsis or slice object.

The pull request I mentioned is gh-4434, is it time to try this? Maybe another note to the list would be good as well before we do that. @llvilanova if you feel strongly about it, please go ahead :).

@llvilanova
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I've just noted it in the list (https://mail.scipy.org/pipermail/numpy-discussion/2015-November/074266.html). Thanks.

@reminisce reminisce mentioned this issue May 30, 2018
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@seberg
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seberg commented Jan 3, 2019

This has a futurewarning now and will eventually change, so closing.

@seberg seberg closed this as completed Jan 3, 2019
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@seberg @llvilanova @njsmith
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