reordered initialization call, added some test code

src/python/dm/common/utility/singleton.py

@@ -4,12 +4,16 @@ class Singleton(object):
 
     __instance = None
 
+    # This class will behave properly as long as getInstance() is called.
+    # If object is constructed using constructor, __init__() will be called
+    # multiple times in the derived class (i.e., one needs protection
+    # against multiple initializations in the derived class)
     def __new__(cls, *args, **kwargs):
         # Allow subclasses to create their own instances.
         if cls.__instance is None or cls != type(cls.__instance):
             instance = object.__new__(cls, *args, **kwargs)
-            cls.__instance = instance
             instance.__init__(*args, **kwargs)
+            cls.__instance = instance
         return cls.__instance
 
     @classmethod
@@ -18,12 +22,13 @@ class Singleton(object):
 
     def __init__(self, *args, **kwargs):
         # Only initialize once.
-        if Singleton.__instance is not None:
+        if self.__class__.__instance is not None:
             return
 
 ####################################################################
 # Testing
 if __name__ == '__main__':
+    print 'Testing Singleton class'
     s1 = Singleton.getInstance()
     s2 = Singleton()
     s3 = Singleton.getInstance()
@@ -33,8 +38,11 @@ if __name__ == '__main__':
     print 'S3: ', s3
     print 'S4: ', s4
     class A(Singleton):
+        __instance = None
         def __init__(self, x):
-            self.x = x
+            if self.__class__.__instance is None:
+                print 'in A.__init__()'
+                self.x = x
     class B(Singleton):
         def __init__(self, x):
             self.x = x
@@ -42,18 +50,31 @@ if __name__ == '__main__':
         def __init__(self):
             self.x = 14
 
+    print
+    print 'Testing Class A'
+    print 'Init A(3)'
     a1 = A(3)
+    print 'Init A(4)'
     a2 = A(4)
+    print 'A.getInstance()'
+    a3 = A.getInstance()
+    print 'A.getInstance()'
+    a4 = A.getInstance()
     print a1
     print a2
-    print a2.x, a1.x
+    print a3
+    print a3.x, a2.x, a1.x
 
+    print
+    print 'Testing Class B'
     b1 = B(6)
     b2 = B(5)
     print b1
     print b2
     print b2.x, b1.x
 
+    print
+    print 'Testing Class C'
     c1 = C()
     c2 = C()
     print c1