Huh? I just checked the Info-ZIP zip man page, and it says (reformatted):

> -g, --grow: Grow (append to) the specified zip archive, instead of creating a new one. If this operation fails, zip attempts to restore the archive to its original state. If the restoration fails, the archive might become corrupted. This option is ignored when there's no existing archive or when at least one archive member must be updated or deleted.

I tested this on Linux and it appears to work as advertised. I assume the reason why it is not enabled by default is because "If the restoration fails, the archive might become corrupted".

edit: python zipfile in append mode also appears to avoid rewriting the zip file. libarchive doesn't seem to support appending to zip files.