[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

Antoine Pitrou added the comment:

I didn't add the 2d example as the double cast is quite ugly.  Probably we 
should allow casting when the format stays the same.

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resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue23632] memoryview doesn't allow tuple-indexing

[Roundup Robot]

Roundup Robot added the comment:

New changeset 7d4eb5902f82 by Antoine Pitrou in branch 'default':
Issue #23632: Memoryviews now allow tuple indexing (including for 
multi-dimensional memoryviews).
https://hg.python.org/cpython/rev/7d4eb5902f82

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nosy: +python-dev

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[issue23632] memoryview doesn't allow tuple-indexing

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

It would be good to add examples with multidimensional memoryviews. For example:

>>> a = array.array('l', [-, , -, ])
>>> m = memoryview(a).cast('B').cast('l', [2, 2])
>>> m[0, 0]
-
>>> m[1, 0]
-
>>> m[-1, -1]


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[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

Antoine Pitrou added the comment:

Updated patch addressing Serhiy's comments.

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Added file: http://bugs.python.org/file38550/memoryview_tuple_indexing2.patch

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[issue23632] memoryview doesn't allow tuple-indexing

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I left few nitpicks on Rietveld, but in general the patch LGTM.

Oh, needed a documentation.

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[issue23632] memoryview doesn't allow tuple-indexing

[Serhiy Storchaka]

Changes by Serhiy Storchaka :


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stage: needs patch -> patch review

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[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

Antoine Pitrou added the comment:

Do you want to review the patch?

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[issue23632] memoryview doesn't allow tuple-indexing

[Stefan Krah]

Stefan Krah added the comment:

It turns out that msg237933 was too simplistic: There are some
intricacies of numpy array indexing that *are* very complex:

  http://docs.scipy.org/doc/numpy/reference/arrays.indexing.html


In particular, x[::2, 1, ::2] is not necessarily equal to
x[::2][1][::2].

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[issue23632] memoryview doesn't allow tuple-indexing

[Stefan Krah]

Stefan Krah added the comment:

Yes, to be clear I'm +1 on this specific feature -- and separate issues. :)

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[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

Antoine Pitrou added the comment:

Well, we could certainly do both :) I don't think we should weigh this issue 
with separate features, though.

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[issue23632] memoryview doesn't allow tuple-indexing

[Stefan Krah]

Stefan Krah added the comment:

sub-views and multi-dimensional slicing are not that bad: They're already 
implemented in _testbuffer (indeed with intermediate objects though).

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[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

Antoine Pitrou added the comment:

Sub-views are not as easily implemented. Besides, they create an intermediate 
object which is immediately thrown away. And tuple indexing is a very common 
idiom in the Numpy universe.

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[issue23632] memoryview doesn't allow tuple-indexing

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

May be add support of multi-dimensional sub-views?

>>> m = memoryview(bytearray(range(12)))
>>> m2 = m.cast('B', (3,4))
>>> m2[0]
Traceback (most recent call last):
  File "", line 1, in 
NotImplementedError: multi-dimensional sub-views are not implemented

Then we could use m2[0][1].

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nosy: +serhiy.storchaka

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[issue23632] memoryview doesn't allow tuple-indexing

[Stefan Krah]

Stefan Krah added the comment:

To be sure, the PEP contains some odd proposals like "Unpacking a long-double 
will return a decimal object", but the one in this issue seems reasonable.

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[issue23632] memoryview doesn't allow tuple-indexing

[Stefan Krah]

Stefan Krah added the comment:

Multi-dimensional slicing is explicitly mentioned in PEP-3118, so
I guess the intention was to cover this use case as well.

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[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

Antoine Pitrou added the comment:

> If we make this work, aren't people going to expect [[1,2], [3,4]][0,1] to 
> work?

Or even [[1,2], [3,4]][*(0,1)] :-)

But seriously, I don't know. memorview is a pretty specialized object, its 
semantics have more to do with Numpy (which has been a major inspiration and 
use case for the buffer protocol's design) than standard Python containers. I 
wouldn't expect a list of lists to support tuple indexing.

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[issue23632] memoryview doesn't allow tuple-indexing

[R. David Murray]

R. David Murray added the comment:

Aren't there wider implications of python starting to support tuple indexing?  
If we make this work, aren't people going to expect [[1,2], [3,4]][0,1] to work?

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nosy: +r.david.murray

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[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

Antoine Pitrou added the comment:

Here is a patch.

--
keywords: +patch
Added file: http://bugs.python.org/file38431/memoryview_tuple_indexing.patch

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[issue23632] memoryview doesn't allow tuple-indexing

[Antoine Pitrou]

New submission from Antoine Pitrou:

It is a bit of pity. A least non-sliced indexing should be able to work:

>>> import numpy as np
>>> a = np.arange(10)
>>> memoryview(a)[0]
0
>>> a = np.arange(10).reshape((2,5))
>>> a[0,1]
1
>>> memoryview(a)[0,1]
Traceback (most recent call last):
  File "", line 1, in 
TypeError: memoryview: invalid slice key

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messages: 237814
nosy: pitrou, skrah
priority: normal
severity: normal
stage: needs patch
status: open
title: memoryview doesn't allow tuple-indexing
type: enhancement
versions: Python 3.5

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