observable_basic.py

from collections import defaultdict

class ObservableAttr:
    class _unset: pass

    def __init__(self, default=_unset):
        self.default = default
        self.name = "unnamed"

    def __set_name__(self, cls, name):
        self.name = name

    def __set__(self, inst, value):
        try:
            inst_attrs = inst.__observable_attrs
        except AttributeError:
            inst_attrs = inst.__observable_attrs = {}
        inst_attrs[self.name] = value
        try:
            listeners = inst.__observable_attrs_listener
        except AttributeError:
            listeners = []
        for listener in listeners[self.name]:
            listener(inst, self.name, value)

    def __get__(self, inst, cls):
        if inst is None:
            return self
        value = inst.__observable_attrs.get(self.name, self.default)
        if value is ObservableAttr._unset:
            raise AttributeError(self.name)
        return value

    def __repr__(self):
        return f"ObservableAttr({self.name})"

    @classmethod
    def observe(cls, inst, attr, listener):
        try:
            listeners = inst.__observable_attrs_listener
        except AttributeError:
            listeners = inst.__observable_attrs_listener = defaultdict(list)
        listeners[attr].append(listener)


class Model:
    foo = ObservableAttr("default foo")
    bar = ObservableAttr("default bar")


def listener(inst, attr, value):
    print(f"{attr} on {inst} set to {value}")

model = Model()
ObservableAttr.observe(model, "foo", listener)
ObservableAttr.observe(model, "bar", listener)

model.foo = "new foo value"
model.bar = "new bar value"
print(f"model.foo {model.foo}")
print(f"model.bar {model.bar}")