def __call__(self, inst, attr, value):
        """
        We use a callable class to be able to change the ``__repr__``.
        """
        if not isinstance(value, self.type):
            raise TypeError(
                "'{name}' must be {type!r} (got {value!r} that is a "
                "{actual!r}).".format(
                    name=attr.name,
                    type=self.type,
                    actual=value.__class__,
                    value=value,
                ),
                attr,
                self.type,
                value,
            )

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