Comment 2 for bug 1767875

This is a safety check added in python-apt to prevent old package objects being reused in new (dep)caches. since the cache operates on package ids, and the package ids might change, there are three things that can happen:

(1) If # of packages in new cache < # in old cache: SEGV
(2) If id changed, functions called on the package might operate on a different package
(3) If you are lucky, everything works