@abstractmethod<EOL><INDENT>def similar_users(self, userid, N=<NUM_LIT:10>):<DEDENT>
pass<EOL>
Calculates a list of similar items

Parameters
----------
userid : int
    The row id of the user to retrieve similar users for
N : int, optional
    The number of similar users to return

Returns
-------
list
    List of (userid, score) tuples