docs: def timetag_str(dt=None, sep='-', filler='0', resolution=6):
      """Generate a date-time tag suitable for appending to a file name.
  
      >>> timetag_str(resolution=3) == '-'.join('{0:02d}'.format(i) for i in
      ...             tuple(datetime.datetime.now().timetuple()[:3]))
      True
      >>> timetag_str(datetime.datetime(2004,12,8,1,2,3,400000))
      '2004-12-08-01-02-03'
      >>> timetag_str(datetime.datetime(2004,12,8))
      '2004-12-08-00-00-00'
      >>> timetag_str(datetime.datetime(2003,6,19), filler='')
      '2003-6-19-0-0-0'
      """
      resolution = int(resolution or 6)
      if sep in (None, False):
          sep = ''
      sep = str(sep)
      dt = datetime.datetime.now() if dt is None else dt
      # FIXME: don't use timetuple which truncates microseconds
      return sep.join(('{0:' + filler + ('2' if filler else '') + 'd}').format(i)
                      for i in tuple(dt.timetuple()[:resolution]))