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.
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