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

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