np.diag()

x = np.arange(9).reshape((3,3))
x
array([[0, 1, 2],
       [3, 4, 5],
       [6, 7, 8]])

np.diag(x)
array([0, 4, 8])
np.diag(x, k=1)
array([1, 5])
np.diag(x, k=-1)
array([3, 7])

np.diag(np.diag(x))
array([[0, 0, 0],
       [0, 4, 0],
       [0, 0, 8]])