When i install ros-melodic on machine 18.04.02, firstly installed catkin by aptitude. Installing catkin will auto install python-rosmsg, python-roslib, ..., etc.
You haven't actually installed Melodic, but an upstreamed version of some of the packages.
Please refer to wiki/UpstreamPackages for more information.
A clear indication is the name of the package: in ROS Melodic, the package would be called ros-melodic-catkin, not catkin.
However, if then i install ros-melodic-desktop-full, catkin, python-rosmsg, python-roslib, etc. will be autoly removed.
yes, and the reason is that those packages (ie: "catkin, python-rosmsg, python-roslib, etc.") are not compatible with ros-melodic-desktop-full, so apt will have to remove them.
(compatibility can actually not be guaranteed, it's not that they are inherently incompatible, but the end result is the same)
You'll want to remove the packages that you installed earlier and then do a proper install of ros-melodic-desktop-full.
