[Numpy-discussion] Initializing array from buffer

[Edison Gustavo Muenz]

Have you tried using the C-API to create the array? This link might be of
help:
http://docs.scipy.org/doc/numpy/reference/c-api.array.html#creating-arrays

I know that Boost.Python can handle this.

On Sun, Nov 16, 2014 at 3:42 PM, Andrea Arteaga <andyspiros at gmail.com>
wrote:

> Hello.
> Using the numpy.frombuffer function [1] one can initialize a numpy array
> using an existing python object that implements the buffer protocol [2].
> This is great, but currently this function supports only 1D buffers, even
> if the provided buffer is multidimensional and it exposes all information
> about its structure (shape, strides, data type).
>
> Apparently, one can extract every kind of buffer information out of a
> buffer of a numpy array (pointer, number of dimensions, shape, strides,
> suboffsets,...), but the other way around is only partially implemented:
> providing a multidimensional buffer does not mean being able of creating a
> numpy array the uses that buffer with the desired structure.
>
> My use case is the following: we have a some 3D arrays in our C++
> framework. The ordering of the elements in these arrays is neither C nor
> Fortran style: it might be IJK (i.e. C style, 3rd dimension contiguous in
> memory), KJI (i.e. Fortran style, first dimension contiguous) or, e.g. IKJ.
> Moreover we put some padding to optimize aligned access. This kind of
> memory structure cannot be just expressed as 'C' or 'Fortran', but it can
> be perfectly expressed using the Python buffer protocol by providing the
> shape and the strides. We would like to export this structure to a numpy
> array that should be able of accessing the same memory locations in a
> consistent way and make some operations like initializing the content or
> plotting it.
>
> Is this currently possible?
> If not, is it planned to implement such a feature?
>
> ==========
>
> Maybe just to clarify I could show an example entirely in python. Assume a
> in a 2D numpy array:
>
> a = np.ones((10,20))
>
> It contains information about its structure which can be portably accessed
> using its data member:
>
> a.data.format == 'd'
> a.data.ndim == 2
> a.data.shape == (10,20)
> a.data.strides == (160,8)
>
> Unfortunately, when initializing an array b from this buffer, the
> structure of the buffer is "downgraded" to unidimensional shape:
>
> b = np.frombuffer(a.data)
>
> b.ndim == 1
> b.shape == (200,)
> b.strides == (8,)
>
> I wished b had the same multi-dimensional structure of a.
>
> (This is of course a very simple example. In my use case I would
> initialize b with my own buffer instead of that of another numpy array).
>
> Best regards
>
> [1]
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.frombuffer.html
> [2] https://docs.python.org/3/c-api/buffer.html
>
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>
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