self._conn = sqlite3.connect(db_file, check_same_thread=False)
 
     def is_registered(self, name):
-        if not db_file: return False
+        if not self._conn: return False
         c = self._conn.cursor()
         c.execute("SELECT COUNT(*) FROM cache WHERE name=?", (name,))
         return c.fetchone() != None
 
     def clean(self):
-        if not db_file: return
+        if not self._conn: return
         c = self._conn.cursor()
         c.execute("DELETE FROM cache WHERE access_time<?", (int(time.time()) - 604800,))
         self._conn.commit()
 
 
     def register(self, name):
-        if not db_file: return
+        if not self._conn: return
         c = self._conn.cursor()
         c.execute( '''INSERT INTO cache (name, access_time) VALUES (?, ?)''',
                    (name, int(time.time())) )
         self._conn.commit()
 
     def refresh(self, name):
-        if not db_file: return
+        if not self._conn: return
         c = self._conn.cursor()
         c.execute( "UPDATE cache SET access_time=? WHERE name=?",
                    (int(time.time()), name) )