Insert an array into another array in numpy?

How do I do this? (in a computationally efficient way)

arr1 = np.array([0, 4, 8, 12, 16])
arr2 = np.array([1, 5, 9, 13, 17])
arr3 = np.array([2, 6, 10, 14, 18])
arr4 = np.array([3, 7, 11, 15, 19])

what_i_want = [ 0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19]

>Solution :

You can directly pass a list of arrays to np.ravel to flatten in Fortran order:

np.ravel([arr1, arr2, arr3, arr4], 'F')

Slower alternative with stack and ravel:

np.stack([arr1, arr2, arr3, arr4]).ravel('F')

output:

array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16,
       17, 18, 19])