<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_resource_attribute(resource_type, resource_id, attr_id, is_var, error_on_duplicate=True, **kwargs):
 """ Add a resource attribute attribute to a resource. attr_is_var indicates whether the attribute is a variable or not -- this is used in simulation to indicate that this value is expected to be filled in by the simulator. """


    attr = db.DBSession.query(Attr).filter(Attr.id==attr_id).first()

    if attr is None:
        raise ResourceNotFoundError("Attribute with ID %s does not exist."%attr_id)

    resource_i = _get_resource(resource_type, resource_id)

    resourceattr_qry = db.DBSession.query(ResourceAttr).filter(ResourceAttr.ref_key==resource_type)

    if resource_type == 'NETWORK':
        resourceattr_qry = resourceattr_qry.filter(ResourceAttr.network_id==resource_id)
    elif resource_type == 'NODE':
        resourceattr_qry = resourceattr_qry.filter(ResourceAttr.node_id==resource_id)
    elif resource_type == 'LINK':
        resourceattr_qry = resourceattr_qry.filter(ResourceAttr.link_id==resource_id)
    elif resource_type == 'GROUP':
        resourceattr_qry = resourceattr_qry.filter(ResourceAttr.group_id==resource_id)
    elif resource_type == 'PROJECT':
        resourceattr_qry = resourceattr_qry.filter(ResourceAttr.project_id==resource_id)
    else:
        raise HydraError('Resource type "{}" not recognised.'.format(resource_type))
    resource_attrs = resourceattr_qry.all()

    for ra in resource_attrs:
        if ra.attr_id == attr_id:
            if not error_on_duplicate:
                return ra

            raise HydraError("Duplicate attribute. %s %s already has attribute %s"
                             %(resource_type, resource_i.get_name(), attr.name))

    attr_is_var = 'Y' if is_var == 'Y' else 'N'

    new_ra = resource_i.add_attribute(attr_id, attr_is_var)
    db.DBSession.flush()

    return new_ra