[Python-ideas] Adding "+" and "+=" operators to dict

[M.-A. Lemburg]

On 12.02.2015 04:59, Chris Angelico wrote:
> On Thu, Feb 12, 2015 at 2:24 PM, Skip Montanaro
> <skip.montanaro at gmail.com> wrote:
>> Addition in the usual sense of the word wouldn't be commutative for
>> dictionaries. In particular, it's hard to see how you could define addition
>> so these two expressions are equal:
>>
>> {'a': 1} + {'a': 2}
>>
>> {'a': 2} + {'a': 1}
>>
>> '+=' is no problem.
> 
> Does it have to be? It isn't commutative for strings or tuples either.
> Addition of complex objects does at times depend on order (though as
> we saw in another thread, it can be very confusing if the _type_ can
> change if you switch the operands), so I would have no problem with
> the right-hand operand "winning" when there's a key collision.

Solving that is simple: you define key collisions as having an
undefined result. Then '+' for dicts is commutative.

However, I don't really see the point in having an operation that
takes two dictionaries, creates a new empty one and updates this
with both sides of the operand. It may be theoretically useful,
but it results in the same poor performance you have in string
concatenation.

In applications, you normally just need the update functionality
for dictionaries. If you do need a copy, you can create a copy
explicitly - but those cases are usually rare.

So +1 on the '+=' syntax, -1 on '+' for dicts.

-- 
Marc-Andre Lemburg
eGenix.com

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