Callbacks not triggered when value updated at the class level before being set on an instance

[maximlt]

Param allows to update the value of instances of a class from the class level, when the value hasn't yet been set (constructor or setattr), e.g.:

import param

class P(param.Parameterized):
    m = param.Parameter(0)
    n = param.Parameter(0)
    o = param.Parameter(0)

p = P(n=1)
p.o = 1
P.m = P.n = P.o = 2
print(f'{p.m=} {p.n=} {p.o=}')
# p.m=2 p.n=1 p.o=1

However, watching a parameter changed at the class level doesn't trigger its callbacks, while it certainly should:

import param

class P(param.Parameterized):
    m = param.Parameter()

    @param.depends('m', watch=True)
    def update(self):
        print(self.m)

p = P()
# Does NOT trigger `update`
P.m = 10
print(p.m)
# 10

[maximlt]

I have a brief (late) look at this issue, I'm not sure how to trigger the callbacks of all the instances of a class when a watched Parameter is updated at the class level. @jbednar @philippjfr interested in your opinion and if it's worth spending time on this to get it in Param 2.0.

[philippjfr]

Yeah, this is awkward to fix. The only way to get all instances would be to keep (weak) references to all instances of a class or iterating over all objects in gc.get_objects(). Both of which seem less than ideal, although I can vaguely imagine the weakref approach. I'd say we skip this one for now.