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

[Bryan]

Terry Reedy wrote:
> 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
[...]
> > However, if you have A inherit from dict...
[...]
> > 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).

I get the same bug-like behavior in 3.1. I think Peter is right that
it's probably a side-effect of an optimization. kkumer seems to have
completely over-ridden the methods of dict, but if we insert into his
hubDict with the parent class's method:

  dict.__setitem__(dh, 'c', 3)

Then the **dh argument passes the keyword arg c=3.


--
--Bryan