intersection, union, difference, symmetric difference for dictionaries
Steven D'Aprano
steve+comp.lang.python at pearwood.info
Wed Feb 26 00:10:29 CET 2014
On Tue, 25 Feb 2014 15:03:51 -0600, Tim Chase wrote:
> On 2014-02-25 14:40, Skip Montanaro wrote:
>> What's the correct result of evaluating this expression?
>>
>> {'A': 1} | {'A': 2}
>>
>> I can see (at least) two possible "correct" answers.
>
> I would propose at least four:
>
> {'A': 1} # choose the LHS
> {'A': 2} # choose the RHS
> {'A': (1,2)} # a resulting pair of both
Should that value be a tuple, a list or a set?
> set(['A']) # you did set-ops, so you get a set
Option 5: raise an exception if the values are different.
Option 6: "or" the values, so that the LHS value is used only if it is
truthy, otherwise the RHS value is used. That is:
{'A': leftdict['A'] or rightdict['A']}
I don't really understand the use-case behind Option 6, but there is a
recent thread on python-ideas at python.org where somebody proposed that as
the Obviously One True And Correct behaviour for dict intersection.
> If dicts were to support set ops, the last one would be my preferred
> result.
What, getting a set back?
No, I disagree. If you want a set, it's easy to do:
dicta.keys() | dictb.keys()
(In Python 2, use viewkeys instead.)
gives you a set of the intersection of the keys. The only point of having
dicts support set-like operations directly is if you get a dict back.
All of these operations are quite simple to write, so personally I would
recommend people just add their own helper function with the behaviour
they prefer.
--
Steven
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