diskimage-builder/diskimage_builder/elements/gentoo/pre-install.d/02-gentoo-04-install-desired-python

#!/bin/bash

if [[ ${DIB_DEBUG_TRACE:-0} -gt 0 ]]; then
    set -x
fi
set -eu
set -o pipefail

if mountpoint -q /dev/shm; then
    echo "/dev/shm found in /proc/self/mountinfo"
elif [[ -k /dev/shm ]]; then
    echo "/dev/shm exists and is stickied"
else
    fix_shm
fi

[[ ! -f ${PORTDIR}/profiles ]] && emerge-webrsync -q

# set active python targets
echo "PYTHON_TARGETS=\"${GENTOO_PYTHON_TARGETS}\"" >> /etc/portage/make.conf
# disable python in git if we have to, it only supports python-2.7
[[ "${GENTOO_PYTHON_TARGETS}" != *"python2_7"* ]] && echo 'dev-vcs/git -python' >> /etc/portage/package.use/git

# install portage first due to bug in env vars not being passed when portage installs itself and re-inits
emerge ${GENTOO_EMERGE_DEFAULT_OPTS} --update --newuse --deep sys-apps/portage
# set the active python version
emerge ${GENTOO_EMERGE_DEFAULT_OPTS} --oneshot --update --newuse --nodeps dev-lang/python-exec dev-lang/python-exec-conf
echo yes | etc-update --automode -3 /etc/python-exec/python-exec.conf

# make world consistant
emerge ${GENTOO_EMERGE_DEFAULT_OPTS} --complete-graph=y --update --newuse --deep @world
# rebuild packages that might need it
emerge ${GENTOO_EMERGE_DEFAULT_OPTS} --usepkg=n @preserved-rebuild
# remove unneeded packages
emerge --verbose=n --depclean
# rebuild packages that might have somehow depended on the unneeded packages
emerge ${GENTOO_EMERGE_DEFAULT_OPTS} --usepkg=n @preserved-rebuild

unfix_shm