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On Mon, Mar 4, 2019 at 3:22 PM Guido van Rossum <guido@python.org> wrote:
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.
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)