Installing ros-melodic-desktop uninstalls python3-catkin-pkg

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Is there a reason this occurs

ROS1 / catkin_make uses python-catkin-pkg to parse package.xml. In ROS2 / colcon python3-catkin-pkg is used.

These 2 packages conflict as they install the same scripts. The libraries can be installed side by side since some modules packages have been created: python3-catkin-pkg-modules and python-catkin-pkg: https://github.com/ros-infrastructure...

is it possible to install a ROS1 distro without removing python3-catkin-pkg

Not that I know of, as these packages conflict. On the bright side I'd expect most of these packages to be reinstalled in their python2 variant along with ROS1. You'll just end up with the python2 version of the executables instead of the python3 one. If I were you I would just run that install and then reinstall the uninstalled modules like python3-rosdep-modules

A better long term resolution would be for packages that use the python modules but not the scripts to declare their dependencies on python(3)-pkg-name-modules instead of python(3)-pkg-name.