[Python-ideas] dict '+' operator and slicing support for pop

[Guido van Rossum]

Because there so many different ways to think about this, it's better
not to guess and force the user to be explicit.

On Tue, Mar 17, 2009 at 9:03 AM, George Sakkis <george.sakkis at gmail.com> wrote:
> On Tue, Mar 17, 2009 at 11:00 AM, Andrii V. Mishkovskyi
> <mishok13 at gmail.com> wrote:
>
>> 1. Add ability to use '+' operator for dicts
>>
>> I often wonder why list and tuple instances have '+' and '+='
>> operators but dicts don't?
>> It's not that rare in my code (and code written by others, as it
>> seems) that i have to write:
>>
>> a.update(b)
>> return a
>>
>> I do understand that adding additional magic method may be
>> inappropriate for dict, but I think it would be nice addition to a
>> language. So, my proposal is that:
>>
>> x = a + b
>> would become equivalent to
>> x = dict(a, **b)
>>
>> a += b
>> would become equivalent to
>> a.update(b)
>
> That's one way to define dict addition but it's not the only, or even,
> the best one. It's hard to put in words exactly why but I expect "a+b"
> to take into account the full state of the operands, not just a part
> of it. In your proposal the values of the first dict for the common
> keys are effectively ignored, which doesn't seem to me as a good fit
> for an additive operation. I would find at least as reasonable and
> intuitive the following definition that doesn't leak information:
>
> def sum_dicts(*dicts):
>    from collections import defaultdict
>    s = defaultdict(list)
>    for d in dicts:
>        for k,v in d.iteritems():
>            s[k].append(v)
>    return s
>
>>>> d1 = {'a':2,'b':5}
>>>> d2 = {'a':2,'c':6,'z':3}
>>>> d3 = {'b':2,'c':5}
>>>> sum_dicts(d1,d2,d3)
> defaultdict(<type 'list'>, {'a': [2, 2], 'c': [6, 5], 'b': [5, 2], 'z': [3]})
>
> George
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)