
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.
* 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
-- --Guido van Rossum (python.org/~guido)
--
--- You received this message because you are subscribed to a topic in the Google Groups "python-ideas" group. To unsubscribe from this topic, visit https://groups.google.com/d/topic/python-ideas/zfHYRHMIAdM/unsubscribe. To unsubscribe from this group and all its topics, send an email to python-ideas+unsubscribe@googlegroups.com. For more options, visit https://groups.google.com/d/optout. _______________________________________________ Python-ideas mailing list Python-ideas@python.org https://mail.python.org/mailman/listinfo/python-ideas Code of Conduct: http://python.org/psf/codeofconduct/
--
--- You received this message because you are subscribed to a topic in the Google Groups "python-ideas" group. To unsubscribe from this topic, visit https://groups.google.com/d/topic/python-ideas/zfHYRHMIAdM/unsubscribe. To unsubscribe from this group and all its topics, send an email to python-ideas+unsubscribe@googlegroups.com. For more options, visit https://groups.google.com/d/optout.