initial creation

py3interm/EXAMPLES/metaclass_singleton.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python
+
+class Singleton(type): # <1>
+    _instances = {}  # <2>
+
+    def __new__(typ, *junk):
+        # print("__new__()")
+        return super().__new__(typ, *junk)
+
+    def __call__(cls, *args, **kwargs):  # <3>
+        # print("__call__()")
+        if cls not in cls._instances:    # <4>
+            cls._instances[cls] = super().__call__(*args, **kwargs)  # <5>
+
+        return cls._instances[cls]   # <6>
+
+
+class ThingA(metaclass=Singleton):   # <7>
+    def __init__(self, value):
+        self.value = value
+
+
+class ThingB(metaclass=Singleton):   # <7>
+    def __init__(self, value):
+        self.value = value
+
+
+ta1 = ThingA(1)  # <8>
+ta2 = ThingA(2)
+ta3 = ThingA(3)
+
+tb1 = ThingB(4)
+tb2 = ThingB(5)
+tb3 = ThingB(6)
+
+for thing in ta1, ta2, ta3, tb1, tb2, tb3:
+    print(type(thing).__name__, id(thing), thing.value)  # <9>