ROS melodic installation with Python 3 only and without messing up system libraries?

Question

Description of the procedure and issues:

I am following the installation guide there:
http://wiki.ros.org/melodic/Installation/Ubuntu

but:

$ sudo apt-get install ros-melodic-desktop-full doesn't work:

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 ros-melodic-desktop-full : Depends: ros-melodic-perception but it is not going to be installed    
E: Unable to correct problems, you have held broken packages.

-> question 1.

$ sudo apt-get install ros-melodic-desktop-full seems to work but it needs to install a whole bunch of packages (602 MB) - which I expected a little. When looking at these packages in details I can see:
- More than 190 ros-something-packages: OK
- A few new libraries: OK
- More than 40 python-<packages> (which are definitely Python 2 packages): -> question 2.1
- More than 80 libboost-<something-system-libraries>: -> question 2.2

Related questions:

Question 1:

Do you know why?

Question 2.1:

How to force the usage of the Python 3 version of these packages?

Question 2.2:

For other purposes I build boost from sources ( Building Boost from sources on Linux ).
Hence I fear that if I install ros with all these dependencies, the boost system libraries that come along with ros (which are all boost version 1.65) will mess up with my personalized installation of boost libraries (which are in version 1.68), especially when I would like to compile other softwares (I prefer to only have one version of these lib; i.e. the latest 1.68).
Is there a way to tell ros to use these already installed libraries (basically in /usr/local/)?

General informations:

I'm using Ubuntu 18.04: 4.15.0-43-generic x86_64 GNU/Linux Python 3 version is: 3.6.7

Mohammad Ali Mohammad Ali · Accepted Answer · 2019-02-02T15:46:38

In order for ROS Systems to works, it Needs dependencies at a minimum specific version.

You cannot use python 3 in installation since all tools are made with python 2.7+ But you can specify what python version to use when running or compiling packages By changing ROS_PYTHON_VERSION to 3.

I think you can use This Answer to convert your compiled source to a .deb file and install it over to prevent apt from interfering with your boost libraries