01.03.19 12:44, Steven D'Aprano пише:
On Fri, Mar 01, 2019 at 08:47:36AM +0200, Serhiy Storchaka wrote:
Currently Counter += dict works and Counter + dict is an error. With
this change Counter + dict will return a value, but it will be different
from the result of the += operator.
That's how list.__iadd__ works too: ListSubclass + list will return a
value, but it might not be the same as += since that operates in place
and uses a different dunder method.
Why is it a problem for dicts but not a problem for lists?
Because the plus operator for lists predated any list subclasses.
Also, if the custom dict subclass implemented the plus operator with
different semantic which supports the addition with a dict, this change
will break it, because dict + CustomDict will call dict.__add__ instead
of CustomDict.__radd__.
That's not how operators work in Python or at least that's not how they
worked the last time I looked: if the behaviour has changed without
discussion, that's a breaking change that should be reverted.
You are right.
What's wrong with doing this?
new = type(self)()
Or the equivalent from C code. If that doesn't work, surely that's the
fault of the subclass, the subclass is broken, and it will raise an
exception.
Try to do this with defaultdict.
Note that none of builtin sequences or sets do this. For good reasons
they always return an instance of the base type.
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