{DataArray,Dataset} accessors with parameters

I would like to be able to create an DataArray accessor that takes parameters, e.g. obj.weighted(w).sum(dim). This appears to be impossible using the existing @register_{dataarray,dataset}_accessor, which supports only accessors of the form obj.weighted.sum(w, dim).

To support the desired syntax, one could simply change https://github.com/pydata/xarray/blob/master/xarray/core/extensions.py#L36
from

            accessor_obj = self._accessor(obj)

to

            accessor_obj = partial(self._accessor, obj)

But that would break the current syntax (i.e. would require obj.accessor().foo(), so is clearly not acceptable.

So any suggestions (short of simply creating slightly modified copies of register_{dataarray,dataset}_accessor) for supporting both the existing obj.accessor.foo() syntax as well as my desired obj.accessor(*args, **kwargs).foo() syntax?

Start in xarray/core/extensions.py at the accessor registration logic referenced in the issue. Investigate how to support both obj.accessor.foo() and obj.accessor(*args, **kwargs).foo() without breaking the existing syntax; done means both forms work for DataArray and Dataset accessors.