not end in a newline, then treat
        # it as a partial line.  We only check for '\n' here because a line
        # ending with '\r' might be a line that was split in the middle of a
        # '\r\n' sequence (see bugs 1555570 and 1721862).
        if not parts[-1].endswith('\n'):
            self._partial.write(parts.pop())
        self.pushlines(parts)

    def pushlines(self, lines):
        self._lines.extend(lines)

    def __iter__(self):
        return self

    def __next__(self):
        line = self.readline()
        if line == '':
            raise StopIteration
        return line