ZIP files can be created to transfer it via unseekable streams (pipes, sockets). Mercurial uses a workaround to write ZIP files right to wsgirequest, but this is possible only with writestr(). write() needs seek() to updated file size, compressed sized and CRC. However ZIP file format supports streamed data without writing sizes and CRC before file data. It is possible and desirable to add full support of unseekable output files in zipfile.