Returning the class

Remember the decorator sane_equals that you defined:

def sane_equals(cls):
    def __eq__(self, other):
        return vars(self) == vars(other)
    cls.__eq__ = __eq__
    return cls  # ?!

Why do you need to return the class if the assignment cls.__eq__ = __eq__ modifies the class in place? You know the answer…

The return statement is very important because decorator syntax is just syntactic sugar for a reassignment into the same name! So, even though plugging the method __eq__ modifies the class in place, if you don’t return it, you’ll overwrite the class with the useless value None:

def class_decorator(cls):
    pass  # Do nothing, but return nothing

@class_decorator
class C:
    pass

print(C)  # None