Examples of metaprogramming in pure Python.

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diff --git a/Demo/metaclasses/Enum.py b/Demo/metaclasses/Enum.py
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+"""Enumeration metaclass."""
+
+import string
+
+class EnumMetaClass:
+    """Metaclass for enumeration.
+
+    To define your own enumeration, do something like
+
+    class Color(Enum):
+	red = 1
+	green = 2
+	blue = 3
+
+    Now, Color.red, Color.green and Color.blue behave totally
+    different: they are enumerated values, not integers.
+
+    Enumerations cannot be instantiated; however they can be
+    subclassed.
+
+    """
+
+    def __init__(self, name, bases, dict):
+	"""Constructor -- create an enumeration.
+
+	Called at the end of the class statement.  The arguments are
+	the name of the new class, a tuple containing the base
+	classes, and a dictionary containing everything that was
+	entered in the class' namespace during execution of the class
+	statement.  In the above example, it would be {'red': 1,
+	'green': 2, 'blue': 3}.
+
+	"""
+	for base in bases:
+	    if base.__class__ is not EnumMetaClass:
+		raise TypeError, "Enumeration base class must be enumeration"
+	bases = filter(lambda x: x is not Enum, bases)
+	self.__name__ = name
+	self.__bases__ = bases
+	self.__dict = {}
+	for key, value in dict.items():
+	    self.__dict[key] = EnumInstance(name, key, value)
+
+    def __getattr__(self, name):
+	"""Return an enumeration value.
+
+	For example, Color.red returns the value corresponding to red.
+
+	XXX Perhaps the values should be created in the constructor?
+
+	This looks in the class dictionary and if it is not found
+	there asks the base classes.
+
+	The special attribute __members__ returns the list of names
+	defined in this class (it does not merge in the names defined
+	in base classes).
+
+	"""
+	if name == '__members__':
+	    return self.__dict.keys()
+
+	try:
+	    return self.__dict[name]
+	except KeyError:
+	    for base in self.__bases__:
+		try:
+		    return getattr(base, name)
+		except AttributeError:
+		    continue
+
+	raise AttributeError, name
+
+    def __repr__(self):
+	s = self.__name__
+	if self.__bases__:
+	    s = s + '(' + string.join(map(lambda x: x.__name__,
+					  self.__bases__), ", ") + ')'
+	if self.__dict:
+	    list = []
+	    for key, value in self.__dict.items():
+		list.append("%s: %s" % (key, int(value)))
+	    s = "%s: {%s}" % (s, string.join(list, ", "))
+	return s
+
+
+class EnumInstance:
+    """Class to represent an enumeration value.
+
+    EnumInstance('Color', 'red', 12) prints as 'Color.red' and behaves
+    like the integer 12 when compared, but doesn't support arithmetic.
+
+    XXX Should it record the actual enumeration rather than just its
+    name?
+
+    """
+
+    def __init__(self, classname, enumname, value):
+	self.__classname = classname
+	self.__enumname = enumname
+	self.__value = value
+
+    def __int__(self):
+	return self.__value
+
+    def __repr__(self):
+	return "EnumInstance(%s, %s, %s)" % (`self.__classname`,
+					     `self.__enumname`,
+					     `self.__value`)
+
+    def __str__(self):
+	return "%s.%s" % (self.__classname, self.__enumname)
+
+    def __cmp__(self, other):
+	return cmp(self.__value, int(other))
+
+
+# Create the base class for enumerations.
+# It is an empty enumeration.
+Enum = EnumMetaClass("Enum", (), {})
+
+
+def _test():
+
+    class Color(Enum):
+	red = 1
+	green = 2
+	blue = 3
+
+    print Color.red
+    print dir(Color)
+
+    print Color.red == Color.red
+    print Color.red == Color.blue
+    print Color.red == 1
+    print Color.red == 2
+
+    class ExtendedColor(Color):
+	white = 0
+	orange = 4
+	yellow = 5
+	purple = 6
+	black = 7
+
+    print ExtendedColor.orange
+    print ExtendedColor.red
+
+    print Color.red == ExtendedColor.red
+
+    class OtherColor(Enum):
+	white = 4
+	blue = 5
+
+    class MergedColor(Color, OtherColor):
+	pass
+
+    print MergedColor.red
+    print MergedColor.white
+
+    print Color
+    print ExtendedColor
+    print OtherColor
+    print MergedColor
+
+if __name__ == '__main__':
+    _test()