On Oct 22, 2019, at 11:39, Mike Miller <python-ideas@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.

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