Cast to a pointer to a Cython extension class

Question

I'm new to Cython so bear with me. I'm trying to cast some data to a pointer to an extension class. The class is essentially a glorified struct. It is declared in my .pxd file as:

cdef class Field:

    cdef:
        np.float64_t u                   # x velocity
        np.float64_t v                   # y velocity
        np.float64_t w                   # z velocity

    cdef update(self)

And of course this is implemented in a .pyx file. In my driver code, I have a 4-dimensional array of np.float64_t's. The first three dimensions represent x, y, and z. The fourth dimension is supposed to represent these three values u, v, w. I allocate the grid in a pure Python driver program which then passes the grid to a Cython file. In the Cython file, I cast:

curr_grid_element = (<Field *> &grid[xx, yy, zz, 0])
curr_grid_element.update()

But when I do so, I get the error: Pointer base type cannot be a Python object.

Which has me rather confused, as I thought Field was pure C.

Answer 1

Cython extension types do not translate into c structures, they translate to python built in types. At a high level this means Field is a python class which happens to be implemented in C, like list or ndarray for example. At a lower level, cython does use c structures to implement extension types but these structures have fields you are not accounting for. Namely a field for tracking its type and anther for reference counting (maybe some others too).

I think what you want is struct, not an extension type. Try this:

cdef struct Field:
    np.float64_t u                   # x velocity
    np.float64_t v                   # y velocity
    np.float64_t w                   # z velocity