[Numpy-discussion] PyArray_BASE equivalent in python

[Peter Rennert]

I probably did something wrong, but it does not work how I tried it. I 
am not sure if you meant it like this, but I tried to subclass from 
ndarray first, but then I do not have access to __array_interface__. Is 
this what you had in mind?

from PySide import QtGui
import numpy as np

class myArray():
     def __init__(self, shape, bits, strides):
         self.__array_interface__ = \
             {'data': bits,
              'typestr': '<i32',
              'descr': [('', '<f8')],
              'shape': shape,
              'strides': strides,
              'version': 3}

image = QtGui.QImage('/home/peter/code/pyTools/sandbox/images/faceDemo.jpg')

b = myArray((image.width(), image.height()), image.bits(), 
(image.bytesPerLine(), 4))
b = np.asarray(b)

b.base
#<read-write buffer ptr 0x7fd744c4b010, size 1478400 at 0x264e9f0>

del image

b
# booom #

On 11/26/2013 08:12 PM, Peter Rennert wrote:
> Brilliant thanks, I will try out the "little class" approach.
>
> On 11/26/2013 08:03 PM, Nathaniel Smith wrote:
>> On Tue, Nov 26, 2013 at 11:54 AM, Peter Rennert <p.rennert at cs.ucl.ac.uk> wrote:
>>> Hi,
>>>
>>> I as the title says, I am looking for a way to set in python the base of
>>> an ndarray to an object.
>>>
>>> Use case is porting qimage2ndarray to PySide where I want to do
>>> something like:
>>>
>>> In [1]: from PySide import QtGui
>>>
>>> In [2]: image =
>>> QtGui.QImage('/home/peter/code/pyTools/sandbox/images/faceDemo.jpg')
>>>
>>> In [3]: import numpy as np
>>>
>>> In [4]: a = np.frombuffer(image.bits())
>>>
>>> --> I would like to do something like:
>>> In [5]: a.base = image
>>>
>>> --> to avoid situations such as:
>>> In [6]: del image
>>>
>>> In [7]: a
>>> Segmentation fault (core dumped)
>> This is a bug in PySide -- the buffer object returned by image.bits()
>> needs to hold a reference to the original image. Please report a bug
>> to them. You will also get a segfault from code that doesn't use numpy
>> at all, by doing things like:
>>
>> bits = image.bits()
>> del image
>> <anything involving the bits object>
>>
>> As a workaround, you can write a little class with an
>> __array_interface__ attribute that points to the image's contents, and
>> then call np.asarray() on this object. The resulting array will have
>> your object as its .base, and then your object can hold onto whatever
>> references it wants.
>>
>> -n
>> _______________________________________________
>> NumPy-Discussion mailing list
>> NumPy-Discussion at scipy.org
>> http://mail.scipy.org/mailman/listinfo/numpy-discussion
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion at scipy.org
> http://mail.scipy.org/mailman/listinfo/numpy-discussion