[Cython] how to allow access to data in a C array?

[Stefan Behnel]

Hi,

Chris Colbert wrote:
> I'm thinking this may be what the new array type is for, but i'm not sure.
> 
> say I have a C struct which I wrap as a class:
> 
> 
> #mycfile.h
> struct Foo
> {
>        int* data[32];
> };
> 
> #my_dec_file.pxd
> cdef exter from "mycfile.h":
> 
>      cdef struct Foo:
>            int* data[32]
> 
> #my_cy_file.pxd
> cimport my_dec_file
> 
> cdef class Foo:
>        cdef my_dec_file.Foo* thisptr
> 
> #my_cy_file.pyx
> cimport my_dec_file
> 
> cdef class Foo:
>     property data:
>     def _get_(self):
>          return self.thisptr.data   <----i dont think this will work

Right, it won't.


> what I want to be able to do in python is this:
> 
> <....get an instance of Foo  i.e. foo = Foo() ....>
> 
> b = foo.data[3:6]   # where b can be a numpy array or a list, but a
> numpy array would be better
> 
> Further, is there a way to manipulate data like that directly via
> python (i.e. without making a copy).
> 
> suppose I want to add the value 2 to every value in foo.data[:]
> 
> it would be nice to do (from python):
> 
> foo.data[:] + 2
> 
> Is there a way to implement that on the Cython side?

Sure, just return an object from the "data" property that wraps the array
and implements __setitem__ and __getitem__ appropriately. Not exactly
trivial, but not too hard either.

Once Cython has better array support, I guess this would simply become a
builtin feature.

Stefan