[Numpy-discussion] Ensuring one can operate on array-like argument in place

Chris Barker chris.barker at noaa.gov
Mon Nov 14 18:20:53 EST 2016


I tend to use ndarray.copy() in python code -- no reason you couldn't do
the same in Cython.

If you want to take any array-like object that may not have a copy()
method, you could call asanyarray() first:

-CHB





On Sat, Nov 12, 2016 at 9:00 AM, Pavlyk, Oleksandr <
oleksandr.pavlyk at intel.com> wrote:

> Hi,
>
>
>
> In my Cython code a function processes it argument x as follows:
>
>
>
>     x_arr = PyArray_CheckFromAny(
>
>           x, NULL, 0, 0,
>
>           cnp.NPY_ELEMENTSTRIDES | cnp.NPY_ENSUREARRAY |
> cnp.NPY_NOTSWAPPED, NULL)
>
>
>
>     if x_arr is not x:
>
>        in_place = 1  # a copy was made, so we can work in place.
>
>
>
> The logic is of the last line turns out to be incorrect, because the input
> x can be a class with an array interface:
>
>
>
> class FakeArray(object):
>
>     def __init__(self, data):
>
>         self._data = data
>
>         self.__array_interface__ = data.__array_interface__
>
>
>
> Feeding my function FakeArray(xx),  x_arr will point into the content of
> xx, resulting in unwarranted content
> overwrite of xx.
>
>
>
> I am trying to replace that condition with
>
>
>
>     if x_arr is not x and cnp.PyArray_Check(x):
>
>        # a copy was made, so we can work in place.
>
>        in_place = 1 if cnp.PyArray_CHKFLAGS(x_arr, cnp.NPY_WRITEABLE) else
> 0
>
>
>
> I am wondering if I perhaps overlooked some case.
>
>
>
> Thank you,
>
> Sasha
>
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>
>


-- 

Christopher Barker, Ph.D.
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