How to bloom-generate with python3

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Hi, I have an Ubuntu installation with ROS Melodic, and I am trying to build my packages using bloom. ROS Melodic is running on python2, but the python code in my package is running on python3.

I can catkin_make my package with the argument -DPYTHON_EXECUTABLE="/usr/bin/python3", but if I run bloom-generate and then fakeroot debian/rules binary, the build fails. I solve this by manually adding the argument -DPYTHON_EXECUTABLE="/usr/bin/python3" in override_dh_auto_configure in the debian/rules file, and now my build succeeds.

Is there a way to add this line automatically, using bloom?

The full (modified) debian/rules file is as follows:

#!/usr/bin/make -f
# -*- makefile -*-
# Sample debian/rules that uses debhelper.
# This file was originally written by Joey Hess and Craig Small.
# As a special exception, when this file is copied by dh-make into a
# dh-make output file, you may use that output file without restriction.
# This special exception was added by Craig Small in version 0.37 of dh-make.

# Uncomment this to turn on verbose mode.
export DH_VERBOSE=1
# TODO: remove the LDFLAGS override.  It's here to avoid esoteric problems
# of this sort:
#  https://code.ros.org/trac/ros/ticket/2977
#  https://code.ros.org/trac/ros/ticket/3842
export LDFLAGS=
export PKG_CONFIG_PATH=/opt/ros/melodic/lib/pkgconfig
# Explicitly enable -DNDEBUG, see:
#   https://github.com/ros-infrastructure/bloom/issues/327
export DEB_CXXFLAGS_MAINT_APPEND=-DNDEBUG
ifneq ($(filter nocheck,$(DEB_BUILD_OPTIONS)),)
    BUILD_TESTING_ARG=-DBUILD_TESTING=OFF -DCATKIN_ENABLE_TESTING=OFF
endif

DEB_HOST_GNU_TYPE ?= $(shell dpkg-architecture -qDEB_HOST_GNU_TYPE)

%:
    dh $@ -v --buildsystem=cmake --builddirectory=.obj-$(DEB_HOST_GNU_TYPE)

override_dh_auto_configure:
    # In case we're installing to a non-standard location, look for a setup.sh
    # in the install tree that was dropped by catkin, and source it.  It will
    # set things like CMAKE_PREFIX_PATH, PKG_CONFIG_PATH, and PYTHONPATH.
    if [ -f "/opt/ros/melodic/setup.sh" ]; then . "/opt/ros/melodic/setup.sh"; fi && \
    dh_auto_configure -- \
        -DCATKIN_BUILD_BINARY_PACKAGE="1" \
        -DCMAKE_INSTALL_PREFIX="/opt/ros/melodic" \
        -DCMAKE_PREFIX_PATH="/opt/ros/melodic" \
        -DPYTHON_EXECUTABLE="/usr/bin/python3" \
        $(BUILD_TESTING_ARG)

override_dh_auto_build:
    # In case we're installing to a non-standard location, look for a setup.sh
    # in the install tree that was dropped by catkin, and source it.  It will
    # set things like CMAKE_PREFIX_PATH, PKG_CONFIG_PATH, and PYTHONPATH.
    if [ -f "/opt/ros/melodic/setup.sh" ]; then . "/opt/ros/melodic/setup.sh"; fi && \
    dh_auto_build

override_dh_auto_test:
    # In case we're installing to a non-standard location, look for a setup.sh
    # in the install tree that was dropped by catkin, and source it.  It will
    # set things like CMAKE_PREFIX_PATH, PKG_CONFIG_PATH, and PYTHONPATH.
    echo -- Running tests. Even if one of them fails the build is not canceled.
    if [ -f "/opt/ros/melodic/setup.sh" ]; then . "/opt/ros/melodic/setup.sh"; fi && \
    dh_auto_test || true

override_dh_shlibdeps:
    # In case we're installing to a non-standard location, look for a setup.sh
    # in the install tree that was dropped by catkin, and source it.  It will
    # set things like CMAKE_PREFIX_PATH, PKG_CONFIG_PATH, and PYTHONPATH.
    if [ -f "/opt/ros/melodic/setup.sh" ]; then . "/opt/ros/melodic/setup.sh"; fi && \
    dh_shlibdeps -l$(CURDIR)/debian/ros-melodic-my-robot-common//opt/ros/melodic/lib/

override_dh_auto_install:
    # In case ...