Create validators not tied to attributes

Currently if I need to add a validator to check a condition that affects two attributes, I have to make it a validator of one of them. For example:

@attr.s
class C(object):
     x = attr.ib()
     y = attr.ib()

     @x.validator
     def x_smaller_than_y(self, _ , value):
          if value >= self.y:
          raise ValueError("'x' has to be smaller than 'y'!")

This feels a little unintuitive because this validator is not really a validator on just 'x'. It should be a validator on 'x' and 'y'. It made me wonder if the order of writing x and y in the class matters, in case the validator is run right after an attribute is set.

I wonder what folks think about a generic validator of this form:

@attr.s
class C(object):
     x = attr.ib()
     y = attr.ib()

     @validator
     def x_smaller_than_y(self):
          if self.x >= self.y:
          raise ValueError("'x' has to be smaller than 'y'!")

The issue names no files or tests; start by reviewing the existing attribute-bound validator behavior shown through @x.validator. Compare it with the proposed class-level @validator and determine how cross-attribute checks should run. Done means a generic validator can enforce the relationship between x and y without being attached to only one attribute, with the behavior covered by tests.