Return a decoder function.
        If cfg is a string such as 'latin-1' or u'latin-1',
        then we return a new lambda, s.decode().
        If cfg is already a lambda or function, then we return that.

def _decoderFromString(cfg):
        """
        Return a decoder function.
        If cfg is a string such as 'latin-1' or u'latin-1',
        then we return a new lambda, s.decode().
        If cfg is already a lambda or function, then we return that.
        """
        if isinstance(cfg, (bytes, str)):
            return lambda s: s.decode(cfg, 'replace')
        return cfg