import numpy as np

a, b = 1, 1
n = 7
r = 4

y, x = np.ogrid[-a:n-a, -b:n-b]
mascara = x*x + y*y <= r

vetor = np.arange(49).reshape(n, n)
vetor[mascara] = 255

print(vetor)

[[255 255 255   3   4   5   6]
 [255 255 255 255  11  12  13]
 [255 255 255  17  18  19  20]
 [ 21 255  23  24  25  26  27]
 [ 28  29  30  31  32  33  34]
 [ 35  36  37  38  39  40  41]
 [ 42  43  44  45  46  47  48]]

[[255 255 255   3   4   5   6]
 [255  8  255  10  11  12  13]
 [255 255 255  17  18  19  20]
 [ 21  22  23  24  25  26  27]
 [ 28  29  30  31  32  33  34]
 [ 35  36  37  38  39  40  41]
 [ 42  43  44  45  46  47  48]]