How to compute sum over a tuple with unknown length in Cython-CodePudding

I want to understand why the following Cython code fails to compile. This function: dimsum simply computes sum of all dimensions of a numpy array:

from numpy cimport ndarray
import numpy as np

cpdef int dimsum(ndarray x):
    cdef:
        int N
        tuple shape
    
    shape = x.shape
    N = np.sum(shape)

    return N

The dimension of input array: x is unknown before run-time. As such, the shape of input array can not be unpacked and assigned to a few new variables.

I got the following error in the compilation:

Error compiling Cython file:
------------------------------------------------------------
...
cpdef int get_size(ndarray x):
    cdef:
        int N
        tuple shape

    shape = x.shape
            ^
------------------------------------------------------------

test_shape_tuple.pyx:9:13: Cannot convert 'npy_intp *' to Python object

Any suggestions on why this happens and how to resolve this?

CodePudding user response：

Despite your mysterious intent with this function, you can solve this error by assigning the npy_intp* type to the shape variable directly.

A secondary performance-related issue with your code is the use of the np.sum function that forces a conversion to Python object.

A more efficient code would be (note that you could avoid defining the shape variable and reading x.shape directly in the for loop):

import numpy as np
cimport numpy as np
from numpy cimport ndarray, npy_intp


cpdef int dimsum(ndarray x):
    cdef:
        int i, out
        npy_intp* shape
    with nogil:
        shape = x.shape
        out = 0
        for i in range(x.ndim):
            out  = shape[i]
    return out