def __call__(self, inst, attr, value):
        """
        We use a callable class to be able to change the ``__repr__``.
        """
        if not self.compare_func(value, self.bound):
            raise ValueError(
                "'{name}' must be {op} {bound}: {value}".format(
                    name=attr.name,
                    op=self.compare_op,
                    bound=self.bound,
                    value=value,
                )
            )

        We use a callable class to be able to change the ``__repr__``.
        
product/gradle-plugin/src/main/python/attr/validators.py