[Numpy-discussion] ctypes and ndpointer

[Albert Strasheim]

Hello all

Just a quick note on the ndpointer function that Travis recently added to
NumPy (thanks Travis!).

When wrapping functions with ctypes, one can specify the argument types of
the function. ctypes then checks that the parameters are valid before
invoking the C function.

This is described here in detail:

http://docs.python.org/dev/lib/ctypes-specifying-required-argument-types.htm
l

The argtypes list is optional, and I think previously Travis suggested not
specifying the argtypes because it would require one to write something like
this:

bar.argtypes = [POINTER(c_double)]
x = N.array([...])
bar(x.data_as(POINTER(c_double))

instead of simply:

bar(x)

What ndpointer allows one to do is to build classes with a from_param method
that knows about the details of ndarrays and how to convert them to
something that ctypes can send to a C function. For example, suppose you
have the following function:

void bar(int* data, double x);

You know that bar expects a 20x30 array of big-endian integers in Fortran
order. You can make sure it gets only this kind of array by doing:

_foolib = N.ctypes_load_library('foolib_', '.')
bar = _foolib.bar
bar.restype = None
p = N.ndpointer(dtype='>i4', ndim=2, shape=(20,30), flags='FORTRAN')
bar.argtypes = [p, ctypes.c_double]

x = N.zeros((20,30),dtype='>i4',order='F')
bar(x, 123.0)

If you want your function to accept any kind of ndarray, you can do:

bar.argtypes = [N.ndpointer(),...]

In this case it will probably still make sense to wrap the C function in a
Python function that also passes the .ctypes.strides and .ctypes.shape of
the array.

Cheers,

Albert

P.S. Sidebar: do we want these ctypes functions in the top-level namespace?
Maybe not. Also, I'm starting to wonder whether ctypes_load_library deserves
to exist or whether we should hear from the ctypes guys if there is a better
way to accomplish what it does (which is to make it easy to load a shared
library/DLL/dylib relative to some file in your module on any platform).