Singleton Patterns

Author: MilovanTomasevic

src/1_patterns/2_CreationalPatterns/4_Singleton/1_singleton_allocator.py

@@ -0,0 +1,30 @@
+import random
+
+class Database:
+    initialized = False
+
+    def __init__(self):
+        # self.id = random.randint(1,101)
+        # print('Generated an id of ', self.id)
+        # print('Loading database from file')
+        pass
+
+    _instance = None
+
+    def __new__(cls, *args, **kwargs):
+        if not cls._instance:
+            cls._instance = super(Database, cls)\
+                .__new__(cls, *args, **kwargs)
+
+        return cls._instance
+
+
+database = Database()
+
+if __name__ == '__main__':
+    d1 = Database()
+    d2 = Database()
+
+    print(d1.id, d2.id)
+    print(d1 == d2)
+    print(database == d1)

src/1_patterns/2_CreationalPatterns/4_Singleton/2_singleton_decorator.py

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+def singleton(class_):
+    instances = {}
+
+    def get_instance(*args, **kwargs):
+        if class_ not in instances:
+            instances[class_] = class_(*args, **kwargs)
+        return instances[class_]
+
+    return get_instance
+
+
+@singleton
+class Database:
+    def __init__(self):
+        print('Loading database')
+
+
+if __name__ == '__main__':
+    d1 = Database()
+    d2 = Database()
+    print(d1 == d2)

src/1_patterns/2_CreationalPatterns/4_Singleton/3_singleton_metaclass.py

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+class Singleton(type):
+    """ Metaclass that creates a Singleton base type when called. """
+    _instances = {}
+
+    def __call__(cls, *args, **kwargs):
+        if cls not in cls._instances:
+            cls._instances[cls] = super(Singleton, cls)\
+                .__call__(*args, **kwargs)
+        return cls._instances[cls]
+
+
+class Database(metaclass=Singleton):
+    def __init__(self):
+        print('Loading database')
+
+
+if __name__ == '__main__':
+    d1 = Database()
+    d2 = Database()
+    print(d1 == d2)