On Mon, Mar 4, 2019 at 12:12 PM Neil Girdhar <mistersheik@gmail.com> wrote:
On Mon, Mar 4, 2019 at 2:26 PM Guido van Rossum <guido@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.
> * 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.

--Guido van Rossum (python.org/~guido)