Fix copying when having watchers on e.g. bounds on inherited Parameter types

[ovidner]

When exposing a parameter with Panel, it might be watched for changes in its bounds. Suppose you later want to make a deepcopy of this parameter. This will work fine if the parameter is a Number, but not if it is e.g. an Integer, per this example:

import param
import copy


class A(param.Parameterized):
    n = param.Number(default=0)
    i = param.Integer(default=0)


obj_1 = A()
obj_1.param.watch(lambda x: x, 'n', 'bounds')
copy.deepcopy(obj_1)  # Works!

obj_2 = A()
obj_2.param.watch(lambda x: x, 'i', 'bounds')
copy.deepcopy(obj_2)  # Raises AttributeError!

As I see it, this issue is due to Parameter.__setattr__ only checking for slot attributes in the class in question itself, and not its superclasses:

param/param/parameterized.py

Line 1120 in 10fff77

slot_attribute = attribute in self.__slots__

In the case of Integer, the bounds attribute is part of __slots__ in Number, but not in Integer, later resulting in the error above.

Proposed solution

Use get_all_slots to traverse up the class hierarchy and see if the attribute is part of any superclass' __slots__.

This is the shortest fix I could think of, but should this rather be handled by calling the superclasses' __setattr__ a few rows down?

param/param/parameterized.py

Line 1133 in 10fff77

super(Parameter, self).__setattr__(attribute, value)

I know too little about the nuances of Param to tell. However, the approach does not seem to break any tests.

I would love to have this solved, so please chime in on how we can do it the best way possible 😃

[philippjfr]

The reasoning and the fix make sense to me. @jbednar could you also take a quick look?

[jbednar]

Looks like the right fix to me, thanks!