[Python-Dev] PEP 450 adding statistics module

[Oscar Benjamin]

On 9 September 2013 12:56, Nick Coghlan <ncoghlan at gmail.com> wrote:
>> Alternatively, I thought there was discussion a long time ago about
>> getting numpy's
>> (or even further back, numeric's?) array type into the core. Python
>> has an array type
>> which I don't think gets a lot of use (or love). Might it be
>> worthwhile to make sure the
>> PEP 450 package works with that? Then extend it to multiple dimensions? Or
>> just
>> bite the bullet and get numpy's array type into the Python core once
>> and for all?
>>
>> Sort of Tulip for arrays...
>
> Aka memoryview :)
>
> Stefan Krah already fixed most of the multidimensional support issues in 3.3
> (including the "cast" method to reinterpret the contents in a different
> format). The main missing API elements are multidimensional slicing and the
> ability to export them from types defined in Python.

Being very familiar with numpy's ndarrays and not so much with
memoryviews this prompted me to go and have a look at them.

How exactly are you supposed to create a multidimensional array using
memoryviews? The best I could come up with was something like:

$ py -3.3
Python 3.3.2 (v3.3.2:d047928ae3f6, May 16 2013, 00:03:43) [MSC v.1600
32 bit (Intel)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import array
>>> v = memoryview(array.array('b', [1, 2, 3, 4])).cast('b', (2, 2))
>>> v.shape
(2, 2)

However I don't seem to be able to access the elements:

>>> v[0, 1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: memoryview: invalid slice key
>>> v[0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NotImplementedError: multi-dimensional sub-views are not implemented
>>> v[0:1]
<memory at 0x00DF8B70>
>>> v[0:1].shape
(1, 2)
>>> v[0:1][0]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NotImplementedError: multi-dimensional sub-views are not implemented
>>> v[1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NotImplementedError: multi-dimensional sub-views are not implemented
>>> v[:, 1]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: memoryview: invalid slice key
>>> v[1, :]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: memoryview: invalid slice key

And the .cast method bails if you try to use a more useful type code:

>>> v = memoryview(array.array('q', [1, 2, 3, 4])).cast('q', (2, 2))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: memoryview: cannot cast between two non-byte formats


Oscar