I like the proposal of adding an operator but I dislike the usage of "+". I'd expect this to do a recursive merge on the dict values for duplicate keys (i.e. adding the values), even more so since `Counter` (being a subclass of dict) already has that behavior. I understand that "+" is meant as a shorthand for `update` and this is what `Counter` does but what sticks more to the mind is the resulting behavior.
Furthermore, since this operation is potentially lossy, I think it would be helpful if the associated operator is not a symmetric symbol but instead is explicit about which operand takes precedence for conflicting keys. The lshift "<<" operator, for example, does have this property. It would be pretty clear what this means `a << b`: take the items of "b" and put them into "a" (or a copy thereof, overwriting what's already there) in order to create the result. The PEP mentions lack of interest in this operator though, as well as: > The "cuteness" value of abusing the operator to indicate information flow got > old shortly after C++ did it. I think a clear advantage of "<<" over "+" is that it indicates the direction (or precedence) which is important if items are potentially to be overwritten. I'd say "old but gold". In the section about [Dict addition is lossy](https://www.python.org/dev/peps/pep-0584/#dict-addition-is-lossy) you write that "no other form of addition is lossy". This is true for the builtin types (except for floating point accuracy) but as part of the stdlib we have `collections.deque` which supports "+" and can be lossy if it specifies `maxlen`. For example: >>> d1 = deque([1, 2], maxlen=3) >>> d2 = deque([3, 4]) >>> d1 + d2 deque([2, 3, 4], maxlen=3) I think this is unfortunate especially since as a double ended queue it supports both `extend` and `extendleft`, so it's not clear whether this extends d1 by d2 or left-extends d2 by d1 (though the latter would probably be ambiguous about the order of items appended). Usage of `d1 << d2` on the other hand would be explicit and clear about the direction of data flow. Although a bit different for dicts, it would as well indicate which of the operands takes precedence over the other. _______________________________________________ Python-ideas mailing list -- python-ideas@python.org To unsubscribe send an email to python-ideas-le...@python.org https://mail.python.org/mailman3/lists/python-ideas.python.org/ Message archived at https://mail.python.org/archives/list/python-ideas@python.org/message/6A3DW3UI6BQGI7H4YWT62JWZOKETZLKL/ Code of Conduct: http://python.org/psf/codeofconduct/
