human
Solve the following python programming problem given the following code:
def _fetch_rrd_meta(self, connection, rrd_path_root, whitelist, field_names) : 
    ' Fetch metadata about each RRD in this Cacti DB, returning a list of
            tuples of (hostname, device_name, rrd_path)
        ' 
    def _in_whitelist(rrd) : 
        path = rrd.replace ('<path_rra>/', '') 
        for p in whitelist : 
            if fnmatch (path, p) : 
                return True 
return False 
c = connection.cursor () 
    and_parameters = ' OR '.join ([("hsc.field_name = '%s'" % field_name) for field_name in field_names]) 
    rrd_query = ("
            SELECT
                h.hostname as hostname,
                hsc.field_value as device_name,
                dt.data_source_path as rrd_path
            FROM data_local dl
                JOIN host h on dl.host_id = h.id
                JOIN data_template_data dt on dt.local_data_id = dl.id
                LEFT JOIN host_snmp_cache hsc on h.id = hsc.host_id
                    AND dl.snmp_index = hsc.snmp_index
            WHERE dt.data_source_path IS NOT NULL
            AND dt.data_source_path != ''
            AND (%s OR hsc.field_name is NULL) " % and_parameters) 
    c.execute (rrd_query) 
    res = [] 
    for (hostname, device_name, rrd_path) in c.fetchall () : 
        if ((not whitelist) or _in_whitelist (rrd_path)) : 
            if (hostname in ('localhost', '127.0.0.1')) : 
                hostname = self.hostname 
rrd_path = rrd_path.replace ('<path_rra>', rrd_path_root) 
            device_name = (device_name or None) 
            res.append ((hostname, device_name, rrd_path)) 
num_hosts = len (set ([num_hosts [0] for r in res])) 
    self.gauge ('cacti.rrd.count', len (res)) 
    self.gauge ('cacti.hosts.count', num_hosts) 
    return res 

    
    

    Fix the buggy line: num_hosts = len (set ([num_hosts [0] for r in res]))
assistant
#Here is Python code to fix the buggy line: num_hosts = len (set ([num_hosts [0] for r in res]))
.

num_hosts = len (set ([r [0] for r in res]))