BFS for iw prefix path

gistfile1.py

    def interwiki_prefix_path(self, site, max_depth=2):                         
        # all sites already checked                                             
        visited = set()                                                         
        old_queue = [(self, [])]                                                
        new_queue = []                                                          
        for depth in range(max_depth):                                          
            print('D{}'.format(depth))                                          
            print('Q{}'.format(old_queue))                                      
            for checked_site, path in old_queue:                                
                print('-' * 30)                                                 
                print('S{} P{}'.format(checked_site, path))                     
                try:                                                            
                    prefix = checked_site.interwiki_prefix(site)                
                except KeyError:                                                
                    for prefix, cached in checked_site._iw_sites.items():           
                        print('P{} S{} L{}'.format(prefix, cached[0], cached[1]))
                        if cached[1] and cached[0] not in visited:              
                            old_queue += [(cached[0], path + [prefix])]         
                            visited.add(cached[0])                              
                else:                                                           
                    return path + [prefix]                                      
            old_queue.sort(key=lambda e: 0 if e[0].family != self.family else 1)
            old_queue, new_queue = new_queue, []