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

[Guido van Rossum]

Honestly I would rather withdraw the subtraction operators than reopen the
discussion about making dict more like set.

On Mon, Mar 4, 2019 at 12:33 PM Neil Girdhar <mistersheik at gmail.com> wrote:

> 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)
>


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