OK, thanks for explaining! So more formally speaking, you want to say that for other examples of '+' in Python x1 + y == x2 + y if and only if x1 == x2, while for the proposed '+' for dicts there may be many different x_i such that x_i + y gives the same result.

It's bit different thank my mind. I'm OK to violate " x1 + y == x2 + y if and only if x1 == x2", if it's not important for semantics of type of x1, x2, and y. Mapping is defined by key: value pairs. It's core part. I don't want to call operator losts key: value pair as "sum". That's why I thought this proposal is more serious abuse of + operator.

By the way, in case of sequence, `len(a) + len(b) == len(a + b)`. In case of set, `len(a) + len(b) >= len(a | b)`. Proposed operation looks similar to `set | set` than `seq + seq` in this point of view.

I don't propose | than +. I just mean difference between dict.update() and seq+seq is not smaller than difference between dict.update() and set|set.

If | seems not fit to this operation, + seems not fit to this operation too.