On Mon, Mar 4, 2019 at 2:26 PM Guido van Rossum <[email protected]> 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.
> * 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.
>
> AFAICT this will give the expected result for defaultdict -- it keeps the
> default factory from the left operand (i.e., self).
>
> * Regarding how often this is needed, we know that this is proposed and
> discussed at length every few years, so I think this will fill a real need.
>
> * Regarding possible anti-patterns that this might encourage, I'm not aware
> of problems around list + list, so this seems an unwarranted worry to me.
>
I agree with these points.
Best,
Neil
> --
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