I'm not crazy about advertising APIs this way ("did you mean ..."), and
even if we would eventually decide to do this, I'm not sure that dict+dict
is the place to start. (Okay, we already started, with "print x" saying
"Did you mean print(x)?" -- but that shows how rare this should be IMO.)Anyway, __add__ should return NotImplemented in the else branch, to give C.__radd__ a chance in the case dict()+C() where C does not subclass dict. I *think* it should then be safe but there are some traps here (e.g. __radd__ sometimes gets called before __add__, if the right operand's class is a subclass of the left operand's class). On Tue, Oct 22, 2019 at 3:18 PM Andrew Barnert via Python-ideas < python-ideas@python.org> wrote: > On Oct 22, 2019, at 11:39, Mike Miller <python-id...@mgmiller.net> wrote: > > > > Had an idea, why not choose the more accurate syntax: |, |= after all? > Then, to help newcomers and forgetful pros a custom error message is > implemented for +, +=. In pseudo C/Python, something like this: > > > > class dict: > > > > def __add__(self, other): > > > > if isinstance(other, dict): > > raise TypeError( > > 'unsupported operand type(s) for +: … ' > > 'Dictionary merging leads to last-value-wins data ' > > 'loss. If acceptable, use the union "|" operator.' > > ) > > else: > > raise TypeError(std_err_msg) > > > > This seems nifty—but will it break the __radd__ protocol? In other words: > > class FancyDict(dict): > def __add__(self, other): > # handles other being a plain dict just fine > def __radd__(self, other): > # handles other being a plain dict just fine > > … you want to make sure that adding a dict (or other dict subclass) and a > FancyDict in either order calls the FancyDict method. > > Off the top of my head, I think it’s safe—and if not it would be safe to > move your logic to dict.__radd__ and have __add__ either not there or > return NotImplemented, because there’s a rule that if one object is an > instance of a subclass of the other object’s type it gets first dibs. But > someone needs to read the data model docs carefully—and also check what > happens if both types are C extensions (since dict is). > > Anyway, while I don’t know if there is precedent for anything like this in > a builtin type’s methods, there is precedent in builtin functions, like > sum, so I think if it’s doable it might be acceptable. The only question is > whether you’d want the same error for adding instances of subclasses of > dict that don’t override the method(s)—and I think the answer there is yes, > you would. > > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/5N67XBNGAEQK2OF3XALT4TPUTMLJYNUF/ > Code of Conduct: http://python.org/psf/codeofconduct/ > -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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