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Add symmetric_difference to Counter #138682
Description
Activity
- added a commit that references this issue
on Sep 10, 2025 I’m concerned about naming this operation
Counter.__xor__. People expect^to be an associative operation, as it is for integers and sets, yet now>>> x = Counter(a=1) >>> y = Counter(a=10) >>> z = Counter(a=100) >>> (x ^ y) ^ z Counter({'a': 91}) >>> x ^ (y ^ z) Counter({'a': 89})
There’s a similar but different way to generalize
set.__xor__that behaves more like^and would be associative, namely the one that takes the bitwise XOR of each count:Counter({item: x[item] ^ y[item] for item in x | y if x[item] != y[item]})
I don’t propose implementing that operation at this time. But can we consider renaming the subtraction-based operation from this PR to something other than
__xor__for Python 3.15, such asCounter.symmetric_differenceorCounter.absolute_difference?
Proposal
This was recently proposed on discuss.python.org and was also proposed 15 years ago.
The old proposal did not move forward because:
Since that time, some of those reasons have changed:
Also looking at the discussions, the most obvious workaround
(p | q) - (p & q)has the disadvantage of not working with signed inputs. A better workaround would be(p - q) | (q - p)which always gives the difference between the maximum count and the minimum count regardless of sign.At this point, I'm inclined to complete the multiset API by adding
symmetric_difference. With all the other multiset methods now being supported, there is a greater cognitive load for the method being missing than for it being present. It is easier to teach that all set operators are supported than teaching that all but one are supported.The only downsides I can think of are:
p ^ qwould need to think carefully about what it does.Linked PRs