Python 3 users in ROS Melodic and earlier: note, if you are building ROS from source to achieve Python 3 compatibility, and have setup your system appropriately (ie: have the Python 3 versions of all the required ROS Python packages installed, such as catkin) the first catkin_make command in a clean catkin workspace must be: $ catkin_make -DPYTHON_EXECUTABLE=/usr/bin/python3 This will configure catkin_make with Python 3. You may then proceed to use just catkin_make for subsequent builds.