custom error-on-duplicate set collection

class ErrorOnDuplicateSet(object):
	"""
	This class behaves just like a set type collection, but will error if
	a duplicate is added to the collection.
	"""

	__emulates__ = set

    def __init__(self):
        self.data = set()

    @collection.appender
    def append(self, item):
    	try:
    		assert item not in data
    	except:
    		raise
    	else:
        	self.data.add(item)

    def remove(self, item):
        self.data.remove(item)

    def __iter__(self):
        return iter(self.data)