Steven D'Aprano writes: > Floats violate several of the fundamental rules of mathematics, > e.g. addition is not commutative: AFAIK it is. > py> 1e19 + (-1e19 + 0.1) == (1e19 + -1e19) + 0.1 > False This is a failure of associativity, not commutativity. Associativity is in many ways a more fundamental property.