Circular binary structure for SciPy morphological operations

I wanted to try buffering a mask raster, without converting to a shapefile and then back, so I used the SciPy Multi-dimensional Image Module's binary_dilation function. I then used the following function to generate a circular binary structure array of the desired radius:

import numpy as np

def circular_structure(radius):
    size = radius*2+1
    i,j = np.mgrid[0:size, 0:size]
    i -= (size/2)
    j -= (size/2)
    return np.sqrt(i**2+j**2) <= radius