custom set collection behavior

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()

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

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

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

	def __repr__(self):
		return str(self.data)


# The allows the following behavior (which I belive is what I should want):

>>> from app.models import User
>>> u = User(email='append_test@email.com', password='password')
>>> u.sz_shirt_dress_sleeve
set()
>>> from app.models import SizeKeyShirtDressSleeve
>>> size_obj = SizeKeyShirtDressSleeve.query.filter(SizeKeyShirtDressSleeve.size == 3000).first()
>>> u.sz_shirt_dress_sleeve.append(size_obj)
>>> u.sz_shirt_dress_sleeve
{Dress shirt sleeve size: 3000}
>>> 


# And errors appropriately (I think? I don't have a best practice for generating custom errors.)

>>> u.sz_shirt_dress_sleeve.append(size_obj)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/lirum/Envs/ltf-nZ8b7LIQ/lib/python3.6/site-packages/sqlalchemy/orm/collections.py", line 995, in wrapper
    return method(*args, **kw)
  File "/Users/lirum/Documents/programming/projects/ltf/app/models.py", line 20, in append
    assert item not in self.data
AssertionError
>>>