import sys

class FooBase(object):
	def __new__(cls, *arguments, **keyword):
		for subclass in FooBase.__subclasses__():
			if subclass.platform == sys.platform:
				return super(cls, subclass).__new__(subclass, *arguments, **keyword)
		raise Exception, 'Platform not supported'

	def say_foo(self):
		print 'foo'

	def say_platform_foo(self):
		raise NotImplementedError

class WindowsFoo(FooBase):
	platform = 'win32'

	def say_platform_foo(self):
		print 'win32 foo'

class LinuxFoo(FooBase):
	platform = 'linux2'

	def say_platform_foo(self):
		print 'linux foo'

if __name__ == '__main__':
	foo = FooBase()
	foo.say_platform_foo()
	def say_platform_foo(self):
		print 'linux foo'