prerm is in libpython2.7-minimal 2.7.15~rc1-1.
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 22 23 24 25 26 27 28 29 30 31 32 33 | #! /bin/sh
set -e
remove_bytecode()
{
    pkg=$1
    dpkg -L $1 \
	| awk '/\.py$/ {print $0"c\n" $0"o"}' \
	| xargs -r rm -f >&2
}
case "$1" in
    remove)
	pc=$(dpkg-query -f '${db:Status-Abbrev} ${binary:Package}\n' -W libpython2.7-minimal \
            | grep -v '^.n' | wc -l)
	if [ "$pc" -le 1 ]; then
            remove_bytecode libpython2.7-minimal:amd64
	fi
        ;;
    upgrade)
	remove_bytecode libpython2.7-minimal:amd64
        # byte compilation in python2.7-minimal postinst, strict dependency
        ;;
    deconfigure)
        ;;
    failed-upgrade)
        ;;
    *)
        echo "prerm called with unknown argument \`$1'" >&2
        exit 1
    ;;
esac
 |