Acces all off diagonal elements of boolean numpy matrix

Question

Suppose there is a diagonal matrix M:

#import numpy as np

M = np.matrix(np.eye(5, dtype=bool))

Does anybody know a simple way to access all off diagonal elements, meaning all elements that are False? In R I can simply do this by executing

M[!M]

Unfortunately this is not valid in Python.

Answer 1

You need the bitwise not operator:

M[~M]

Answer 2

You might try np.extract combined with np.eye. For example:

M = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
np.extract(1 -  np.eye(3), M)
# result: array([2, 3, 4, 6, 7, 8])

In your example it's almost an identity:

M = np.matrix(np.eye(5, dtype=bool))
np.extract(1 - np.eye(5), M)
#result: 
array([False, False, False, False, False, False, False, False, False,
   False, False, False, False, False, False, False, False, False,
   False, False], dtype=bool)