Not operator, seemingly wrong in Python?

Question

I am new to python and created a small function that does a cluster analysis. The quick rundown is I have to compare two arrays a multitude of times, until it no longer changes. For that I have used a while loop, that loops as long as they are not equal, but I find that I get two different results from != and not ==. MWE:

import numpy as np

a = np.array([1,1,1])
b = np.array([1,2,1])

print((a != b).all())
print(not (a == b))

Answer 1

not (a == b) will raise a ValueError because the truth-value of an array with multiple elements is ambiguous.

The way you invert a boolean array in numpy is with the ~ operator:

>>> a != b
array([False,  True, False], dtype=bool)
>>> ~ (a == b)
array([False,  True, False], dtype=bool)
>>> (~ (a == b)).all() == (a != b).all()
True