For the record, I'm opposed to the idea. * Use of the + operator is a temptation to produce new dictionaries rather than update an existing dict in-place which is usually what you want. * We already have ChainMap() which presents a single view of multiple mappings with any copying. * It is natural to expect the plus operator to be commutative, but this operation would necessarily be non-commutative. * Many other APIs are modeled on the dict API, so we should not grow the API unless there is a big win. The effects would be pervasive. * I don't see other languages going down this path, nor am I seeing dict subclasses that implement this functionality. Those are indications that this more of a "fun thing we could do" rather than a "thing that people need". * The existing code already reads nicely: options.update(user_selections) That reads more like self explanatory English than: options += user_selections The latter takes more effort to correctly parse and makes it less clear that you're working with dicts. * It isn't self-evident that the right operand needs to be another dictionary. If a person is trying to "add a key / value pair" to an existing dictionary, the "+=" operator would be tempting but it wouldn't work.