def only(self):
if len(self) == <NUM_LIT:1>:<EOL><INDENT>return self[<NUM_LIT:0>]<EOL><DEDENT>raise ValueError("<STR_LIT>".format(self))<EOL>
Returns the first item in this list, and asserts that it is the only
item.  This is useful when querying a collection for a resource and
expecting to get only one back.  For instance::

   # raises if it finds more than one Linode
   production_box = client.linode.instances(Linode.group == "prod").only()

:returns: The first and only item in this list.
:raises ValueError: If more than one item is in this list.