@classmethod
def from_spec(cls, spec, validator_map, resource_roots, package='sahara'):
    """Constructs and returns a validator from a specification object.

        :param spec: The specification for the validator.
        :param validator_map: A map of validator name to class.
        :param resource_roots: The roots from which relative paths to
            resources (scripts and such) will be referenced. Any resource will
            be pulled from the first path in the list at which a file exists.
        :return: A validator built to the specification.
        """
    pass
