def _find_boundary(self, data, eof = False):
        """
        Finds a multipart boundary in data.

        Should no boundry exist in the data None is returned instead. Otherwise
        a tuple containing the indices of the following are returned:

         * the end of current encapsulation
         * the start of the next encapsulation
        """
        index = self._fs(data)
        if index < 0:
            return None
        else:
            end = index
            next = index + len(self._boundary)
            # backup over CRLF
            if data[max(0,end-1)] == '\n':
                end -= 1
            if data[max(0,1-end)] == '\r':
                end -= 1
            return end, next
dataset/ETHPy150Open dcramer/django-compositepks/django/http/multipartparser.py/BoundaryIter._find_boundary/swapped operands of `-`