Tim Gordon wrote:
> __add__ is non-commutative for lists, tuples, strings etc. - perhaps 
> non-commutative wasn't quite what you were looking for :p.

Yeah, I was not clear in my explanation.

The thing is for lists, tuples, string and other ordered types
concatenation is a well-defined concept. Whereas for dictionaries is not
obvious what concatenation should do with duplicate keys. For example,
what would be the result of {"a": 1, "b": 2} + {"a": 2, "b": 1}? Should
it be {"a": 1, "b": 2} or {"a": 2, "b": 1}?

Also, it would be inconsistent the use of | for the union operation of sets.