I understand hash randomization was added after this bug was opened. Here is a demonstration with “video/mp4”, which only has the extension “.mp4” built in. But my /etc/mime.types file lists “mp4 mp4v mpg4”, so after the second initialization the behaviour changes:

PYTHONHASHSEED=0 python3.5 -c 'from mimetypes import *; print(guess_all_extensions("video/mp4")); init(); print(guess_all_extensions("video/mp4"))'
['.mp4', '.mp4v', '.mpg4']
['.mpg4', '.mp4', '.mp4v']

The first extension is always “.mp4”, regardless of hash randomization, due to the built-in list. But after re-initialization, the first extension depends on the order in the internal dictionary.

Using an ordered dictionary may work as a bug fix, but the whole initialization logic is so complex and it would be good to simplify it in the long term.