Cython: How to pass a numpy memoryview to a nogil cdef function

Question

I would like to send a numpy array (memoryview) to another Cython function.

cdef int function(int[:,:] ref) nogil:
    pass

cpdef test():
    cdef np.ndarray[int, ndim=2] ref = np.full((10,10),0)
    with nogil:
        function(ref)

This compiles okay, but gives me Windows fatal exception: access violation when I execute. This all works okay, if I remove nogil, but I would prefer not to do this if possible.

Thanks

Answer 1

My opinion is that it shouldn't compile, and that you should report the compiling and the exception as a bug to github.com/cython/cython/issues.

Python Object -> memoryview does require the GIL (although memoryview -> memoryview doesn't). Therefore you need to initialize a memoryview outside the nogil block:

cpdef test():
    cdef np.ndarray[int, ndim=2] ref = np.full((10,10),0)
    cdef int[:,:] ref_view = ref
    with nogil:
        function(ref_view)

or move the nogil block into function (if that makes sense in your real code).

There probably isn't a huge advantage to setting the type np.ndarray[int, ndim=2] - the only thing that gives you is faster element indexing (which is also what memoryviews give you).