Allow reversed(memoryview), like memoryview

[PCManticore]

BPO	19078
Nosy	@rhettinger, @ncoghlan, @pitrou, @skrah, @PCManticore
Files	memoryview.patch

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ncoghlan'
closed_at = <Date 2013-10-02.12:07:06.836>
created_at = <Date 2013-09-23.16:27:16.275>
labels = ['interpreter-core', 'type-feature']
title = 'Allow reversed(memoryview), like memoryview'
updated_at = <Date 2013-10-02.12:07:06.834>
user = 'https://github.com/PCManticore'

bugs.python.org fields:

activity = <Date 2013-10-02.12:07:06.834>
actor = 'python-dev'
assignee = 'ncoghlan'
closed = True
closed_date = <Date 2013-10-02.12:07:06.836>
closer = 'python-dev'
components = ['Interpreter Core']
creation = <Date 2013-09-23.16:27:16.275>
creator = 'Claudiu.Popa'
dependencies = []
files = ['31850']
hgrepos = []
issue_num = 19078
keywords = ['patch']
message_count = 12.0
messages = ['198324', '198342', '198348', '198383', '198384', '198516', '198518', '198566', '198612', '198680', '198824', '198829']
nosy_count = 6.0
nosy_names = ['rhettinger', 'ncoghlan', 'pitrou', 'skrah', 'Claudiu.Popa', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue19078'
versions = ['Python 3.4']

[PCManticore]

Hello. The following seems a little weird:

Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600 32 bit (Intel)] on win32
>>> m = memoryview(b'123')
>>> list(m[::-1])
[51, 50, 49]
>>> list(reversed(m))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: object of type 'memoryview' has no len()
>>>

The attached patch allows reversed to be called with memoryviews and it could potentially fix bpo-18690.

[pitrou]

So the dilemma with len() was: does it return the number of bytes, or the number of items?
Given the new memoryview semantics, I'd say it should return the number of items.

[ncoghlan]

Aye, with the 3.3 changes, I think we should continue down the
multi-dimensional array path. I suggest we run with whatever NumPy uses for
ndarray, which I believe is "number of items in the first dimension".

[PCManticore]

So, in the following case, len shouldn't return 4, but the number of items? Also, is it ok to assume that the number of items is
((*shape)[0] * ... * (*shape)[ndims-1])?

>>> x = np.array([[1, 2, 3], [4, 5, 6], [4,5,6], [4,4,4]], np.int32)
>>> x.shape
(4, 3)
>>> x.shape[0] * x.shape[1]
12
>>> len(x)
4
>>> memoryview(x)
<memory at 0x0217C378>
>>> len(memoryview(x))
4
>>> memoryview(x).shape
(4, 3)
>>>

[pitrou]

No, you're right, it should probably return 4.

[skrah]

Yes, len() should return the number of items. +1 for making reversed()
work.

NumPy does this:

>>> x = numpy.array([1,2,3,4,5,6,7,8])
>>> list(reversed(x))
[8, 7, 6, 5, 4, 3, 2, 1]
>>>
>>> x = numpy.array([[1,2,3], [4,5,6]])
>>> list(reversed(x))
[array([4, 5, 6]), array([1, 2, 3])]
>>>

[skrah]

Hmm, I meant: Number of items in the first dimension, thus 4 in
Claudiu's example.

[PCManticore]

For multidimensional arrays it doesn't seem to work (yet).

>>> x = numpy.array([[1,2,3], [4,5,6]])
>>> list(reversed(x))
[array([4, 5, 6]), array([1, 2, 3])]
>>> x.data
<memory at 0x8032d06b8>
>>> list(reversed(x.data))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NotImplementedError: multi-dimensional sub-views are not implemented
>>>

[pitrou]

Stefan, what do you think about Claudiu's patch? Should a test be added to test_buffer as well?

[rhettinger]

Claudiu's patch looks correct.