Added singleton pattern with overriding __new__ method.

alpiii · alpiii · commit bfa3e091bbf2 · 2016-01-21T18:32:51.000+02:00
diff --git a/README.md b/README.md
@@ -20,6 +20,7 @@ __Creational Patterns__:
 | [lazy_evaluation](lazy_evaluation.py) | lazily-evaluated property pattern in Python |
 | [pool](pool.py) | preinstantiate and maintain a group of instances of the same type |
 | [prototype](prototype.py) | use a factory and clones of a prototype for new instances (if instantiation is expensive) |
+| [singleton](singleton.py) | a singleton with overriding __new__ method |
 
 __Structural Patterns__:
 
diff --git a/singleton.py b/singleton.py
@@ -0,0 +1,50 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+
+
+class Singleton(object):
+    """
+    Assume that only one instance should be created of this class and
+    it is counting the users connected to the system.
+    """
+
+    _instance = None
+
+    def __new__(cls, *args, **kwargs):
+        """
+        For each class, creating only one instance.
+        If an instance has been created before, returns the created one.
+        """
+        if not cls._instance:
+            cls._instance = super(Singleton, cls).__new__(cls, *args, **kwargs)
+
+            # your init codes here
+            cls.user_count = 0
+            # end of your init code
+
+        return cls._instance
+
+    def connect(self):
+        self.user_count += 1
+
+    def disconnect(self):
+        self.user_count -= 1
+
+
+if __name__ == '__main__':
+
+    s1 = Singleton()
+    s2 = Singleton()
+
+    if id(s1) == id(s2):
+        print "Same instances"
+    else:
+        print "Different instances"
+
+    s1.connect()  # user count: 1
+    s1.connect()  # user count: 2
+    s2.connect()  # same instance with s1, so user count: 3 for s1 and s2
+    s1.disconnect()  # same instance with s2, so user count: 2 for s1 and s2
+
+    print s1.user_count
+    print s2.user_count
