Changing pickle file for the same object

I put the object together, then do something with it, then detach it again and check that the object is still the same. This may not be the best way to ensure equality, but it is very strict and simple.

However, I am getting an unexpected difference in the pickle file on some machines running python2.7. I used pickletools to parse the resulting files.

The difference lies in the additional PUT code , although these registers are never available. When are PUT statements generated and why are they generated differently when pickle is called twice on the same object?