Method _lookup

Lib/mailbox.py:795–803

Return (start, stop) or raise KeyError.

(self, key=None)

                self._file.close()  # Sync has been done by self.flush() above.

    def _lookup(self, key=None):
        """Return (start, stop) or raise KeyError."""
        if self._toc is None:
            self._generate_toc()
        if key is not None:
            try:
                return self._toc[key]
            except KeyError:
                raise KeyError('No message with key: %s' % key) from None

    def _append_message(self, message):
        """Append message to mailbox and return (start, stop) offsets."""