[Python-ideas] Dict joining using + and +=

[Neil Girdhar]

On Mon, Mar 4, 2019 at 3:22 PM Guido van Rossum <guido at python.org> wrote:
>
> On Mon, Mar 4, 2019 at 12:12 PM Neil Girdhar <mistersheik at gmail.com> wrote:
>>
>> On Mon, Mar 4, 2019 at 2:26 PM Guido van Rossum <guido at python.org> wrote:
>> >
>> > * Dicts are not like sets because the ordering operators (<, <=, >, >=) are not defined on dicts, but they implement subset comparisons for sets. I think this is another argument pleading against | as the operator to combine two dicts.
>> >
>>
>> I feel like dict should be treated like sets with the |, &, and -
>> operators since in mathematics a mapping is sometimes represented as a
>> set of pairs with unique first elements.  Therefore, I think the set
>> metaphor is stronger.
>
>
> That ship has long sailed.

Maybe, but reading through the various replies, it seems that if you
are adding "-" to be analogous to set difference, then the combination
operator should be analogous to set union "|".  And it also opens an
opportunity to add set intersection "&".  After all, how do you filter
a dictionary to a set of keys?

>> d = {'some': 5, 'extra': 10, 'things': 55}
>> d &= {'some', 'allowed', 'options'}
>> d
{'some': 5}

>>
>> > * Regarding how to construct the new set in __add__, I now think this should be done like this:
>> >
>> > class dict:
>> >     <other methods>
>> >     def __add__(self, other):
>> >         <checks that other makes sense, else return NotImplemented>
>> >         new = self.copy()  # A subclass may or may not choose to override
>> >         new.update(other)
>> >         return new
>>
>> I like that, but it would be inefficient to do that for __sub__ since
>> it would create elements that it might later delete.
>>
>> def __sub__(self, other):
>>  new = self.copy()
>>  for k in other:
>>   del new[k]
>> return new
>>
>> is less efficient than
>>
>> def __sub__(self, other):
>>  return type(self)({k: v for k, v in self.items() if k not in other})
>>
>> when copying v is expensive.  Also, users would probably not expect
>> values that don't end up being returned to be copied.
>
>
> No, the values won't be copied -- it is a shallow copy that only increfs the keys and values.

Oh right, good point.  Then your way is better since it would preserve
any other data stored by the dict subclass.
>
> --
> --Guido van Rossum (python.org/~guido)