Which objects are expanded by double-star ** operator?

[Terry Reedy]

On 6/7/2010 6:03 PM, Peter Otten wrote:
> kkumer wrote:
>
>>
>> I have to merge two dictionaries into one, and in
>> a "shallow" way: changing items should be possible
>> by operating either on two parents or on a
>> new dictionary. I am open to suggestions how
>> to do this (values are always numbers, BTW), but
>> I tried to do it by creating a dict-like class that just
>> forwards all calls to the two parent dicts, see below.
>>
>> It works, but one important thing is missing. I
>> am not able to expand new dictionary with
>> double-star operator ** to use it as a
>> set of keyword arguments of a function.
>> I googled a bit, but was unable to find what
>> property must an object have to be correctly
>> treated by **.
>
> The following experiment shows that you only need to implement a keys() and
> __getitem__() method.
>
> $ cat kw.py
> class A(object):
>      def keys(self): return list("ab")
>      def __getitem__(self, key):
>          return 42
>
> def f(**kw):
>      print(kw)
>
> f(**A())
> $ python kw.py
> {'a': 42, 'b': 42}
>
> However, if you have A inherit from dict...
>
> $ cat kwd.py
> class A(dict):
>      def keys(self): return list("ab")
>      def __getitem__(self, key):
>          return 42
>
> def f(**kw):
>      print(kw)
>
> f(**A())
> $ python kwd.py
> {}
>
> it stops working -- probably a side-effect of some optimization.
> So if you change your hubDict's base class from dict to object you should
> get the desired behaviour.

In 2.6, the requirement changed from '(subclass of) dictionary' to 
'mapping' so this is a bit strange. It sort of looks like a bug. I will 
test with 3.1 tomorrow (later today, actually).

tjr