Most of these args are passed directly to BackEnd(). However, default_host 
    is used a bit differently. It should be a (host, port) pair, and may be 
    overridden with cmdline args:
      file.py localhost
      file.py localhost 54545

def build_backend(tasks, default_host=('127.0.0.1', DEFAULT_PORT), *args, 
                  **kw):
    """
    Most of these args are passed directly to BackEnd(). However, default_host 
    is used a bit differently. It should be a (host, port) pair, and may be 
    overridden with cmdline args:
      file.py localhost
      file.py localhost 54545
    """
    host, port = default_host
    if len(sys.argv) > 1:
        host = sys.argv[1]
    if len(sys.argv) > 2:
        port = sys.argv[2]
    
    return BackEnd(tasks, host, port, *args, **kw)