>>> from attr import attrs, attrib, Factory
>>> @attrs
... class SomeClass(object):
...     a_number = attrib(default=42)
...     list_of_numbers = attrib(default=Factory(list))
...
...     def hard_math(self, another_number):
...         return self.a_number + sum(self.list_of_numbers) * another_number
>>> SomeClass(1, [1, 2, 3])
SomeClass(a_number=1, list_of_numbers=[1, 2, 3])