a = {1,2,3}
b = {1,2,3,6,7,8}
len(a) - len(b) == 0
len(a) != len(b)
Subtraction on sets does not work like you're suggesting it should.
I don't understand why you're using that to defend broken notation.
The only reason why we have a|b as union is because a∪b is too hard to type
on most keyboards, and we still are trying to catch up to APL when it comes
to math source code.
I'm not suggesting anything about subtraction on sets. I'm suggesting that + is not appropriate for dictionary merge because it doesn't behave like + on other Python containers.
The context is Python, not other languages. In a language that used + for set union, + would also be fine for dictionary merge.
But Python isn't like that. Strings, tuples, and lists have + and do not have -, and for all of them the + operator does add lengths. Sets have | and -, and those operators do not add or subtract lengths.
