Proper use of numpy recarrays as c structarrays in cython

Question

I would like to use something like structarray in cython, and I would like this structarray to be easily accessible in python, as in cython. Based on a whim, I used repetition using a dtype that looks like the structure I would like to use. Curiously, it just works and allows me to use c structarray, which is above the hood;), is a multiple repetition for the python user.

Here is my example

# This is a "structarray in cython with numpy recarrays" testfile
import numpy as np
cimport numpy as np

# My structarray has nodes with fields x and y
# This also works without packed, but I have seen packed used in other places where people asked similar questions
# I assume that for two doubles that is equivalent but is necessary for in8s in between
cdef packed struct node:
    double x
    double y
# I suppose that would be the equivalent numpy dtype?
# Note: During compilation it warns me about double to float downcasts, but I do not see where
nodetype = [('x' , np.float64),('y', np.float64)]

def fun():
    # Make 10 element recarray
    # (Just looked it up. A point where 1-based indexing would save a look in the docs)
    mynode1 = np.recarray(10,dtype=nodetype)

    # Recarray with cdef struct
    mynode1 = np.recarray(10,dtype=nodetype)

    # Fill it with non-garbage somewhere
    mynode1[2].x=1.0
    mynode1[2].y=2.0

    # Brave: give recarray element to a c function assuming its equivalent to the struct
    ny = cfuny(mynode1[2])
    assert ny==2.0 # works!

    # Test memoryview, assuming type node
    cdef node [:] nview = mynode1
    ny = cfunyv(nview,2)
    assert ny==2.0 # works!

    # This sets the numpy recarray value with a c function the gts a memoryview
    cfunyv_set(nview,5,9.0)
    assert mynode1[5].y==9.0 # alsow works!

    return 0

# return node element y from c struct node
cdef double cfuny(node n):
    return n.y

# give element i from memoryview of recarray to c function expecting a c struct
cdef double cfunyv(node [:] n, int i):
    return cfuny(n[i])

# write into recarray with a function expecting a memoryview with type node
cdef int cfunyv_set(node [:] n,int i,double val):
    n[i].y = val
    return 0

Of course, I am not the first to try this.

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Answer 1

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%%cython
import numpy as np

cdef struct Thing:
    char a
    # 7 bytes padding, double must be 8 byte aligned
    double b

thing_dtype = np.dtype([('a', np.byte), ('b', np.double)])
print('dtype size: ', thing_dtype.itemsize)
print('unpacked struct size', sizeof(Thing))
dtype size:  9
unpacked struct size 16

Answer 2

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