[Python-ideas] Dict joining using + and +=

Serhiy Storchaka storchaka at gmail.com
Mon Mar 4 08:43:48 EST 2019

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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