Pure Python runtime property to expression binding example using descriptors

property.py

#! /usr/bin/env python3
#
# Pure Python runtime property to expression binding example using descriptors
# (see below for usage example).
#
# This Property implementation allows setting a value using lambda expressions.
# The value will be recalculated if any property accessed during lambda evaluation
# changes its value.

class Property:
    _capture_observer = None

    def __init__(self, default_value=None):
        self.default_value = default_value

    def __get__(self, instance, owner=None):
        if self._capture_observer is not None:
            if self.observers_name not in instance.__dict__:
                instance.__dict__[self.observers_name] = set()

            instance.__dict__[self.observers_name].add(self._capture_observer)

        value = instance.__dict__.get(self.value_name)
        return value or self.default_value

    def __set__(self, instance, value_or_func):
        if callable(value_or_func):
            instance.__dict__[self.value_func_name] = value_or_func

            self._capture_observer = (instance, self)
            instance.__dict__[self.value_name] = value_or_func()
            self._capture_observer = None
        else:
            instance.__dict__[self.value_func_name] = None
            instance.__dict__[self.value_name] = value_or_func

        if self.observers_name in instance.__dict__:
            observers = instance.__dict__[self.observers_name]
            if len(observers) > 0:
                notify_observers = observers.copy()
                observers.clear()

                for instance, p in notify_observers:
                    p.update_value(instance)

    def __set_name__(self, owner, name):
        self.value_name = '__property_{}_value'.format(name)
        self.value_func_name = '__property_{}_func'.format(name)
        self.observers_name = '__property_{}_observers'.format(name)

    def update_value(self, instance):
        value_func = instance.__dict__[self.value_func_name]

        self._capture_observer = instance, self
        instance.__dict__[self.value_name] = value_func()
        self._capture_observer = None

class Employee:
    salary = Property()

    def __init__(self, **kwargs):
        for k, v in kwargs.items():
            setattr(self, k, v)

bob = Employee(name='Bob', salary=50)
tom = Employee(name='Tom', salary=lambda: bob.salary * 2)

print("before:")
print("bob's salary:", bob.salary) # 50
print("tom's salary:", tom.salary) # 100

bob.salary = 60

print("after:")
print("bob's salary:", bob.salary) # 60
print("tom's salary:", tom.salary) # 120