[Python] Python fill in elements of large matrix with same value

I have a large matrix where I want to assign the same value(s) to many matrix elements. Is there a more efficient way to do this rather than iterating over each column and row using for loops? Here is a MWE:

import numpy as np

a = 0.5
b = 0.6

M = np.zeros((16,16)) # empty matrix
np.fill_diagonal(M, 0.9) # diagonal elements

for jj in range(0,16): # rows
for kk in range(0,16): # columns
if jj == 0:
if (kk == 1) | (kk == 3):
M[jj,kk] = a
if jj == 3:
if (kk == 0) | (kk == 2):
M[jj,kk] = b
if jj == 5:
if (kk == 4) | (kk == 6):
M[jj,kk] = a
# etc

print(M)

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