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@@ -1019,3 +1019,52 @@ class Enum:
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if hasattr(self, item):
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return 0
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return 1
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+
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+############################################################
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+# class: Singleton
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+# Purpose: This provides a base metaclass for all classes
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+# that require one and only one instance.
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+#
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+# Example: class mySingleton:
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+# __metaclass__ = PythonUtil.Singleton
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+# def __init__(self,...):
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+# ...
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+#
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+# Note: This class is based on Python's New-Style Class
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+# design. An error will occur if a defined class
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+# attemps to inherit from a Classic-Style Class only,
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+# ie: class myClassX:
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+# def __init__(self, ...):
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+# ...
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+#
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+# class myNewClassX(myClassX):
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+# __metaclass__ = PythonUtil.Singleton
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+# def __init__(self, ...):
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+# myClassX.__init__(self, ...)
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+# ...
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+#
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+# This causes problems because myNewClassX is a
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+# New-Style class that inherits from only a
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+# Classic-Style base class. There are two ways
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+# simple ways to resolve this issue.
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+#
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+# First, if possible, make myClassX a
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+# New-Style class by inheriting from object
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+# object. IE: class myClassX(object):
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+#
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+# If for some reason that is not an option, make
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+# myNewClassX inherit from object and myClassX.
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+# IE: class myNewClassX(object, myClassX):
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+############################################################
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+class Singleton(type):
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+ def __init__(cls,name,bases,dic):
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+ super(Singleton,cls).__init__(name,bases,dic)
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+ cls.instance=None
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+ def __call__(cls,*args,**kw):
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+ if cls.instance is None:
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+ cls.instance=super(Singleton,cls).__call__(*args,**kw)
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+ return cls.instance
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+
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+class SingletonError(ValueError):
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+ """ Used to indicate an inappropriate value for a Singleton."""
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+
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jxtaylor