First, be careful - even restrictive - about what you ever run as root. A normal user could not modify things under /usr/lib, and for good reason - it breaks the system.
Second, you can find out what packages contain things in that directory using:
$ dpkg -S /usr/lib/python2.7
python-qgis, python-gdal, python-psycopg2, python-pyspatialite, youtube-dl, virtualbox, duplicity, bzr-git, bzr-builddeb, debconf, ipython, libpython2.7-minimal:i386, libpython2.7-dev:i386, tahoe-lafs, seascope, samba, qbzr, python2.7, python-zope.interface, python-zfec, python-yaml, python-xdg, python-xapian, python-wxversion, python-wxgtk2.8, python-ws4py, python-webob, python-wadllib, python-vipscc, python-utidylib, python-usb, python-urllib3, python-tz, python-twisted, python-twisted-words, python-twisted-web, python-twisted-runner, python-twisted-news, python-twisted-names, python-twisted-mail, python-twisted-lore, python-twisted-core, python-twisted-conch, python-twisted-bin, python-tk, python-tdb, python-talloc, python-support, python-subversion, python-sphinx, python-software-properties, python-six, python-sip, python-simplejson, python-simplegeneric, python-setuptools, python-setools, python-serial, python-sepolicy, python-sepolgen, python-semanage, python-selinux, python-secretstorage, python-scipy, python-samba, python-routes, python-roman, python-requests, python-repoze.lru, python-reportlab, python-reportlab-accel, python-renderpm, python-radare2, python-qt4, python-qt4-gl, python-qscintilla2, python-pyvorbis, python-pytools, python-pysqlite2, python-pyside.qtxml, python-pyside.qtwebkit, python-pyside.qtuitools, python-pyside.qttest, python-pyside.qtsvg, python-pyside.qtsql, python-pyside.qtscript, python-pyside.qtopengl, python-pyside.qtnetwork, python-pyside.qthelp, python-pyside.qtgui, python-pyside.qtdeclarative, python-pyside.qtcore, python-pyside.phonon, python-pyparsing, python-pyopencl, python-pygments, python-pygame, python-pycurl, python-pycryptopp, python-pyaudio, python-pyasn1, python-poppler-qt4, python-ply, python-pkg-resources, python-pivy, python-pip, python-pil, python-pexpect, python-paramiko, python-pam, python-openssl, python-opengl, python-opencv, python-ogg, python-oauthlib, python-oauth, python-numpy, python-ntdb, python-newt, python-nevow, python-networkx, python-netifaces, python-mysqldb, python-musicbrainz, python-mock, python-mechanize, python-markupsafe, python-markdown, python-mako, python-magic, python-lxml, python-libxml2, python-ldb, python-lazr.uri, python-lazr.restfulclient, python-launchpadlib, python-keyring, python-jinja2, python-ipy, python-imaging, python-httplib2, python-html5lib, python-gtk2, python-gst0.10, python-gst0.10-rtsp, python-gpgme, python-gobject-2, python-glade2, python-gi, python-freenect, python-foolscap, python-feedparser, python-fastimport, python-eyed3, python-enchant, python-egenix-mxtools, python-egenix-mxdatetime, python-ecdsa, python-dulwich, python-docutils, python-docopt, python-dnspython, python-distro-info, python-distlib, python-decorator, python-debian, python-dbus, python-dateutil, python-cssutils, python-cssselect, python-crypto, python-configobj, python-colorama, python-collada, python-cherrypy3, python-chardet, python-bzrlib, python-bluez, python-beautifulsoup, python-audit, python-apt, python-apsw, policycoreutils, mercurial, mercurial-common, lsb-release, iotop, hugin-tools, hplip, frescobaldi, libpython2.7:i386, libpython2.7-stdlib:i386, dblatex, cython, cfv, bzr-upload, bzr-search, bzr-pipeline, bzr-loom, bzr-explorer: /usr/lib/python2.7
(Yes, the list is very long.) Knowing that list, we can request those packages to be reinstalled:
$ sudo apt-get install --reinstall `dpkg -S /usr/lib/python2.7 | sed -e s/,//g -e 's/: .*$//'`
I apologise for the very long command line; the sed command here cleans up the output of dpkg to produce only the list of packages we want to reinstall. This method is likely to help with the specific issue you mention, but even having it occur once suggests you're not clear on the consqeuences of other changes. You may want to slow down and learn more about your system's structure.
Things like PYTHON* variables won't help you much unless you have a precisely matching version of Python elsewhere, something we tend to avoid on Linux distributions because we usually have working (albeit limited) package management.
Lastly, I think the question ends up more of a superuser question than stack overflow.
