Bitwise or works at the level of bits, so the question is more like, in: >>> bin(0b011 | 0b010) '0b11' Does the leading 1 come from the first argument

Bitwise or works at the level of bits, so the question is more like, in: >>> bin(0b011 | 0b010) '0b11' Does the leading 1 come from the first argument or the second argument? It isn't really a sensible question for bits, since the 1s are indistinguishable. However the values in the hash table are distinguishable, so it's a valid question. Also, taking an example from another comment: >>> {0} | {False} {0} >>> {False} | {0} {False} This is another example where the union operator is used, but here it picks the first value, rather than the second value.