method to downgrade debian from a unstable tree to stable tree
step 1 put the following content in /etc/apt/preferences.d/defaultPackage: * Pin: release a=stable Pin-Priority: 1001
make your sources.list to stable pointers.
next do sensibly
To Fix Things, work on a stable system and chroot to the broken system
The command “ar” can be used to extract files from debian binary package
export PYTHONPATH=/usr/lib/python2.7 for debian related for showing python module path related.
the above content may not be accurate.