This pattern restricts the instantiation of a class to one object. It is a type of creational pattern and involves only one class to create methods and specified objects.
It provides a global point of access to the instance created.
The following program demonstrates the implementation of singleton class where it prints the instances created multiple times.
class Singleton: __instance = None @staticmethod def getInstance(): """ Static access method. """ if Singleton.__instance == None: Singleton() return Singleton.__instance def __init__(self): """ Virtually private constructor. """ if Singleton.__instance != None: raise Exception("This class is a singleton!") else: Singleton.__instance = self s = Singleton() print s s = Singleton.getInstance() print s s = Singleton.getInstance() print s
The above program generates the following output −
The number of instances created are same and there is no difference in the objects listed in output.