calling multiple times numpy.ctypeslib.as_array(pointer, shape) always uses the same shape (Trac #2079)

[numpy-gitbot]

Original ticket http://projects.scipy.org/numpy/ticket/2079 on 2012-03-13 by trac user pieleric, assigned to unknown.

The latest version of numpy supports numpy.ctypeslib.as_array() for a ctypes.POINTER. However, when using numpy.ctypeslib.as_array() on a pointer multiple times in a row with different shapes, it's always the shape of the first call which is used.

Example:

import ctypes
import numpy
>>> buffer = ctypes.pointer((ctypes.c_uint16 * 100)())
>>> p = ctypes.cast(buffer, ctypes.POINTER(ctypes.c_uint16))
>>> a = numpy.ctypeslib.as_array(p, shape=(100,))
>>> a.shape
(100,)
>>> b = numpy.ctypeslib.as_array(p, shape=(10,10))
>>> b.shape
(100,)

We would expect b.shape == (10,10) .

The bug is in as_array() and prep_pointer(). They are too much a copy-paste of the case of an ndarray: only create the array_interface if it is not yet there. For an ndarray this works fine as the array_interface is saved on the type. For a pointer, it should be updated every time.

So, I propose something like this:

    def prep_pointer(pointer_obj, shape):
        contents = pointer_obj.contents
        dtype = _dtype(type(contents))

        inter = {'version': 3,
                 'typestr': dtype.str,
                 'data': (ct.addressof(contents), False),
                 'shape': shape}

        pointer_obj.__array_interface__ = inter

    def as_array(obj, shape=None):
        if hasattr(obj, 'contents'):
            prep_pointer(obj, shape)
        else:
            tp = type(obj)
            try: tp.__array_interface__
            except AttributeError: prep_array(tp)
        return array(obj, copy=False)

BTW, the description of as_array() should be updated to mention the "shape" parameter instead of the "size" parameter.

[sdy7]

I would fix this if I knew how to.
Can I vote for this bug?

cheers
Soren

[njsmith]

I suppose you can, but votes don't write code so it won't directly affect
much :-). For more visible results, you could figure out how to fix it, or
try to find a friend interested in spending some time on it - perhaps on
the mailing list.
On 22 Mar 2013 15:48, "sdy7" notifications@github.com wrote:

I would fix this if I knew how to.
Can I vote for this bug?

cheers
Soren

—
Reply to this email directly or view it on GitHubhttps://github.com//issues/2671#issuecomment-15304274
.

[tynn]

Is it still of interest to fix this?

I had a look at it and found doing

try: pointer_obj.__array_interface__['shape'] = shape
except AttributeError: pass
except TypeError: pass
else: return

in prep_pointer would also fix this without rewriting the complete code.

This works if pointer_obj.__array_interface__ is a dictionary or something accepting keys. If the TypeError was raised, a return could pass the error handling to np.array like it would have with the original code, while a pass also ensures validity at that point.