[Numpy-discussion] immutable numpy arrays

[Geoffrey Irving]

On Wed, Dec 17, 2008 at 2:24 PM, Robert Kern <robert.kern at gmail.com> wrote:
> On Wed, Dec 17, 2008 at 15:52, Geoffrey Irving <irving at naml.us> wrote:
>> Currently numpy arrays are either writable or unwritable, but
>> unwritable arrays can still be changed through other copies.  This
>> means that when a numpy array is passed into an interface that
>> requires immutability for safety reasons, a copy always has to be
>> made.
>>
>> One way around this would be to add a NPY_IMMUTABLE flag signifying
>> that the contents of the array will never change through other copies.
>
> This is not possible to guarantee. With the __array_interface__, I can
> make a numpy array point at any addressable memory without its
> knowledge. We can even mutate "immutable" str objects, too.

In python __array_interface__ just returns a big integer representing
a pointer which can't be used for anything.  Well-behaved C code has
to be trusted to care when __array_interface__ marks its data as
unwriteable, so that shouldn't be a problem either.

Is there some other way that arbitrary python code could bypass the
NPY_WRITEABLE flag?

Geoffrey