On Mon, Jun 1, 2020 at 8:50 PM Brandt Bucher <brandtbuc...@gmail.com> wrote:
> > These would be mixin methods, not abstract. Set already implements the > various operators based on the abstract methods, it could easily add more > mixin methods which would delegate to the operators. Classes which override > the operators with more efficient versions would automatically get > efficient aliases for free. > > This same idea came up when we added `|` / `|=` to `dict` in 3.8. The > reason we couldn't safely add those to `Mapping`/`MutableMapping` is that > it could break compatibility for virtual subclasses that have been > `register`-ed, and don't actually inherit from the ABC. > > The problem is the same here. OK, I wasn't on this list then, and I haven't found much while searching. I found a brief mention of the problem [here]( https://mail.python.org/archives/list/python-ideas@python.org/message/WX7PNKTOX2RIYKA4G2OC7RD4HHRLUSBA/) without much explanation of why it might be a problem, and a reply saying that it's not. So I may ask some questions that have been resolved already. What do you mean by 'break compatibility'? Registering a virtual subclass doesn't enforce that any methods are implemented, not even the abstract ones. Are virtual subclasses required to implement all the mixin methods? I thought the mixin methods were just a nice conveniece, not a contract.
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