class Test_Defaults:

	def default_factory(self, x=None, type=None):
		if x is None:
			return self.default_by_type(type)
		else:
			return x

	def default_by_type(self, type):

		strategy_operations = {
			"list" : list,
			"dict" : dict,
			"set" : set,
			}

		operation = strategy_operations.get(type.__class__.__name__)
		if operation is None:
			raise ValueError

		return operation()

	def __init__(self):
		self.x = self.default_factory(None, [])

a = Test_Defaults()
b = Test_Defaults()

a.x.append(1)
print(a.x == b.x)	# False as expected