One issue that's come up during PR review with Guido and Serhiy is, when
evaluating `a | b`, whether or not the default implementation should call an
overridden `copy()` method on `a` in order to create an instance of the correct
subclass (rather than a plain-ol' `dict`). For example, `defaultdict` works
correctly without any additional modification:
```
>>> from collections import defaultdict
>>> i = defaultdict(int, {1: 1, 2: 2})
>>> s = defaultdict(str, {2: "2", 3: "3"})
>>> i | s
defaultdict(<class 'int'>, {1: 1, 2: '2', 3: '3'})
>>> s | i
defaultdict(<class 'str'>, {2: 2, 3: '3', 1: 1})
```
So this has immediate benefits for both usability and maintenance: subclasses
only need to override `copy()` to get working `__or__`/`__ror__` behavior.
While this isn't what `list` and `set` do, for example, I argue that `dict` is
more often subclassed, and we shouldn't blindly follow the precedent of their
behavior when designing a new API for `dict`.
We decided to bring the discussion here to get input from a larger audience.
I'm currently +0 on calling `copy()`, but I know Steven feels a bit more
strongly about this than I do:
> I think the standard handling of subclasses in Python builtins is wrong, and
> I don't wish to emulate that wrong behaviour without a really good reason. Or
> at least a better reason than "other methods break subclassing unless
> explicitly overloaded, so this should do so too". Or at least not without a
> fight :-)
The more detailed thread of discussion starts at
https://github.com/python/cpython/pull/12088#issuecomment-582609024 (note that
we are no longer considering calling an overridden `update` method).
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