[Numpy-discussion] PyArray_BASE equivalent in python

[Peter Rennert]

Btw, I just wanted to file a bug at PySide, but it might be alright at 
their end, because I can do this:

from PySide import QtGui

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

a = image.bits()

del image

a
#<read-write buffer ptr 0x7f5fe0034010, size 1478400 at 0x3c1a6b0>


On 11/26/2013 09:37 PM, Peter Rennert wrote:
> 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
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