In Python, the actual equality testing operator is `is`, not `==`. The fundamental property of equality is that equal objects can't be distinguished in any way.

In Python, the actual equality testing operator is `is`, not `==`. The fundamental property of equality is that equal objects can't be distinguished in any way. (Indiscernibility of identicals, aka Leibniz's principle.) xs = [1,2,3] ys = [1,2,3] # are xs and ys equal? ys.append(4) print(xs == ys) # no!