how to define ROS kinetic to use python3 instead of python2.7?

edit

The situation changed slightly over the last two years. You can compile a complete ROS distribution (I tried kinetic) with

catkin build --cmake-args -DPYTHON_VERSION=3.6

assuming you setup a whole core workspace with something like

rosinstall_generator --rosdistro kinetic desktop_full

This works more or less for the core ROS system. I just filed a few patches to make it even work with MoveIt.

However, Dirk's comment is still true. Sadly even in ROS-melodic (which very few people are using already) there is no enforced python3 support. By now, this is simply hilarious. But ROS2 will of course make everything better... :/

I had to fix a few lines of code in a number of packages I use myself (including tf), but they are all straight-forward python2-3 compatibility issues that are well documented.

So everyone, feel free to try it out and file patches if you find more python2-only code somewhere! We are not that far away from Python3 support anymore.

This will likely become obsolete with the release of Noetic, but as I've just had to build a version of Kinetic with Python 3 support (for the Nth time), I figured I'd post the commands I've used here (for myself as well).

This is a more copy-pastable version of @v4hn's answer (but was too long for a comment).

Assumptions:

• OS: Ubuntu Xenial (16.04)
• ROS Kinetic is already installed on that system (the .deb distributed version)

I always use a virtual environment, as it's typically the case that besides ROS, there are Python modules that need to be installed one might not want to "pollute" the system with.

Notes:

1. this only builds a bare-bones ROS Kinetic with Python 3. No Open CV or other 'complex' packages/nodes. For many use-cases this is already quite sufficient though.
2. as such, this is not an answer to the general problem of how to build all of ROS 1 with Python 3 support.

In any case, as follows:

# create a place for our Python3-ROS-Kinetic to live
mkdir $HOME/rospy3_kinetic
cd $HOME/rospy3_kinetic

# make sure these are installed
apt install python3-venv libpython3-dev python-catkin-tools

python3.5 -m venv venv3.5_ros
source venv3.5_ros/bin/activate

# these modules are needed but are not automatically installed
pip3 install wheel empy defusedxml netifaces

# just the standard build-ros-from-source dependencies
pip3 install -U rosdep rosinstall_generator wstool rosinstall

# seed the workspace with a 'ros_base' set of packages
rosinstall_generator --deps --tar --rosdistro=kinetic ros_base > ros_base.rosinstall
wstool init src -j8 ros_base.rosinstall

# the following is somewhat optional: I like to run this to see if
# something obvious is missing, but we can't ask rosdep to 'install'
# things for us: it will try to use apt to install the Python 2 version
# of all dependencies it sees missing.

# this may fail if you already have done this, in that case: ignore
sudo rosdep init
rosdep update

# I use check here to first see what is missing. If missing dependencies are
# serious enough, use 'pip3' to install into the virtual environment for
# Python dependencies, 'apt' for any other dependencies
rosdep check --from-paths src/ -i

# finally build the workspace, while:
#  - disabling testing globally
#  - making sure optimisations are enabled ('Release' build type)
#  - forcing use of Python 3.5
catkin build -DCATKIN_ENABLE_TESTING=0 -DCMAKE_BUILD_TYPE=Release -DPYTHON_VERSION=3.5

At this point source the setup.bash of the workspace. You may choose to configure catkin_tools to create an install space instead of using the devel space directly.

For regular usage you'll need to make sure to have the virtual environment activated, or things will break.

I'd recommend using workspace overlaying for actual development.

Not sure about the previous versions of ROS, but it easy to build ROS packages with python3 in ROS Melodic.

Modify the following file:

/opt/ros/melodic/etc/catkin/profile.d/1.ros_python_version.sh

from

export ROS_PYTHON_VERSION=2

to

export ROS_PYTHON_VERSION=3

Source the above-mentioned file.

source /opt/ros/melodic/etc/catkin/profile.d/1.ros_python_version.sh

Create a new workspace and build.

If you have an existing workspace, delete the devel/, build/ and .catkin_workspace file from your workspace and build again.