artfwo · January 15, 2021 20:27 · Jan 15, 2021
diff --git a/property.py b/property.py
@@ -0,0 +1,76 @@
+#! /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
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