send paramiko logs to a logfile, if they're not already going somewhere

def log_to_file(filename, level=DEBUG):
    """send paramiko logs to a logfile, if they're not already going somewhere"""
    l = logging.getLogger("paramiko")
    if len(l.handlers) > 0:
        return
    l.setLevel(level)
    f = open(filename, 'w')
    lh = logging.StreamHandler(f)
    lh.setFormatter(logging.Formatter('%(levelname)-.3s [%(asctime)s.%(msecs)03d] thr=%(_threadid)-3d %(name)s: %(message)s',
                                      '%Y%m%d-%H:%M:%S'))
    l.addHandler(lh)