[Python-ideas] PEP: Dict addition and subtraction

Jonathan Fine jfine2358 at gmail.com
Wed Mar 6 07:17:40 EST 2019

Ka-Ping Yee wrote:
> len(dict1 + dict2) does not equal len(dict1) + len(dict2), so using the + operator is nonsense.
> len(dict1 + dict2) cannot even be computed by any expression involving +.  Using len() to test the semantics of the operation is not arbitrary; the fact that the sizes do not add is a defining quality of a merge.  This is a merge, not an addition.  The proper analogy is to sets, not lists.

For me, this comment is excellent. It neatly expresses the central
concern about this proposal. I think most us will agree that the
proposal is to use '+' to express a merge operation, namely update.
(There are other merge operations, when there are two values to
combine, such as taking the min or max of the two values.)

Certainly, many of the posts quite naturally use the word merge.
Indeed PEP 584 writes "This PEP suggests adding merge '+' and
difference '-' operators to the built-in dict class."

We would all agree that it would be obviously wrong to suggest adding
merge '-' and difference '+' operators. (Note: I've swapped '+' and
'-'.) And why? Because it is obviously wrong to use '-' to denote
merge, etc.

Some of us are also upset by the use of '+' to denote merge. By the
way, there is already a widespread symbol for merge. It appears on
many road signs. It looks like an upside down 'Y'. It even has merge
left and merge right versions.

Python already has operator symbols '+', '-', '*', '/' and so on. See

Perhaps we should add a merge or update symbol to this list, so that
we don't overload to breaking point the humble '+' operator. Although
that would make Python a bit more like APL.

By the way, Pandas already has a merge operation, called merge, that
takes many parameters. I've only glanced at it.


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