preinst is in python-numpy 1:1.11.0-1ubuntu1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 | #!/bin/sh
# TODO: remove this file after releasing Squeeze
set -e
PYCENTRAL=/usr/bin/pycentral
if [ "$1" = upgrade ]
then
       if dpkg --compare-versions "$2" lt 1:1.4.1-1 ; then
	   if [ -x $PYCENTRAL ]; then
               # explicitly remove old python-central files
               pycentral pkgremove python-numpy
	   fi
	   # be really sure also removing left-over files and dirs
	   for pyversion in 2.4 2.5; do
                if [ -d /usr/lib/python$pyversion/site-packages/multiprocessing/ ]; then
                    rm -fr /usr/lib/python$pyversion/site-packages/numpy*
                fi
            done
       fi
fi
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