Is os.path.expanduser("~/x") equivalent to os.path.abspath(os.path.expanduser("~/x"))?

It depends on what your $HOME points to. On most properly set-up systems (every mainstream Linux distro, OSX and Windows) it'll point to an absolute path, e.g. /home/user or C:/Users/User. But if it's unset, improperly set or even changed manually (export HOME=.), expanduser may result in a relative path, in which case abspath will further change it.

But for most intents and purposes, you can assume that yes, both expressions are equivalent.