The documentation for emulating callable objects with __call__ seems to imply only positional arguments are supported. For instance, it says __call__ is "object.__call__(self[, args…])" and describes:

Called when the instance is “called” as a function; if this method is defined, x(arg1, arg2, ...) is a shorthand for x.__call__(arg1, arg2, ...).

When it should be something like (Not sure exactly what the syntax would be here):
"object.__call__(self[, args…] [,**kwargs...])"

Called when the instance is “called” as a function; if this method is defined, x(arg1, arg2, ..., arg3=arg3) is a shorthand for x.__call__(arg1, arg2, ..., arg3=arg3).