Example showing np.ix_

get_submatrix.py

import numpy as np
import copy

K # shape (N,N)

# Create a list with all the indices of the submatrix
idx = np.arange(N-1) + 1
for i in range(N):
    # when i==0, idx will be [1,2,3,4...]
    # when i==1, idx will become [0,2,3,4...] etc
    if i > 0:
        idx[i] -= 1
    # Construct the submatrix using np.ix_
    sub_K = K[np.ix_(sub_idx,sub_idx)]