Method _ascii_split

Lib/email/header.py:490–510

(self, fws, string, splitchars)

            yield self._maxlen - self._continuation_ws_len

    def _ascii_split(self, fws, string, splitchars):
        # The RFC 2822 header folding algorithm is simple in principle but
        # complex in practice.  Lines may be folded any place where "folding
        # white space" appears by inserting a linesep character in front of the
        # FWS.  The complication is that not all spaces or tabs qualify as FWS,
        # and we are also supposed to prefer to break at "higher level
        # syntactic breaks".  We can't do either of these without intimate
        # knowledge of the structure of structured headers, which we don't have
        # here.  So the best we can do here is prefer to break at the specified
        # splitchars, and hope that we don't choose any spaces or tabs that
        # aren't legal FWS.  (This is at least better than the old algorithm,
        # where we would sometimes *introduce* FWS after a splitchar, or the
        # algorithm before that, where we would turn all white space runs into
        # single spaces or tabs.)
        parts = re.split("(["+FWS+"]+)", fws+string)
        if parts[0]:
            parts[:0] = ['']
        else:
            parts.pop(0)
        for fws, part in zip(*[iter(parts)]*2):
            self._append_chunk(fws, part)

    def _append_chunk(self, fws, string):
        self._current_line.push(fws, string)