参考网址: https://www.cnblogs.com/cymwill/p/7857456.html
# https://www.cnblogs.com/cymwill/p/7857456.html
import numpy as np
arr = np.array([[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16]])
print(np.tril(arr, k=-1)) # Lower triangle of an array.
print(np.triu(arr, k=1)) # Upper triangle of an array