code: def to_bytes(s, encoding="utf-8"):
      
      if isinstance(s, six.binary_type):
          return s
      else:
          return six.text_type(s).encode(encoding)
docs: Converts the string to a bytes type, if not already.
  
      :s: the string to convert to bytes
      :returns: `str` on Python2 and `bytes` on Python3.