Bsrgvty

I am able to run these commands in the terminal:

$ sudo update-alternatives --install /usr/share/gnome-shell/theme/gdm3.css gdm3.css /usr/share/gnome-shell/theme/mytheme/mytheme.css 10
$ sudo update-alternatives --config gdm3.css
There are 2 choices for the alternative gdm3.css (providing /usr/share/gnome-shell/theme/gdm3.css).

 Selection Path Priority Status
------------------------------------------------------------
* 0 /usr/share/gnome-shell/theme/ubuntu.css 10 auto mode
 1 /usr/share/gnome-shell/theme/mytheme/mytheme.css 10 manual mode
 2 /usr/share/gnome-shell/theme/ubuntu.css 10 manual mode

Press <enter> to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/share/gnome-shell/theme/mytheme/mytheme.css to provide /usr/share/gnome-shell/theme/gdm3.css (gdm3.css) in manual mode

I will put these two cmds in python3.6 subprocess.run().

The issue I have is with the second command. How do I assign /usr/share/gnome-shell/theme/mytheme/mytheme.css to be the alternative of gdm3.css in the same cmdline as sudo update-alternatives --config gdm3.css?

Also, this cmd requires a number entry to select mytheme.css. The assigned number is dependent on the assigned priority, which means the assigned number can be quite arbitrary. How do I overcome the arbitrary nature of the assigned number?

Based on @PRATAP comment, I have developed this python3.6 solution to my question. It worked on my Ubuntu 18.04 system. I hope it can benefit others with the same need.

#!/usr/bin/env python3.6
# -*- coding: utf-8 -*-

from subprocess import run, PIPE
from pathlib import Path
import mimetypes

class CSSFileTypeError(Exception):
 pass

class GDM3_alternatives:
 '''Class to query and configure gdm3.css.

 Argument:
 mytheme - path to my gnome-shell theme .css file.

 Attributes:
 mytheme - path to my gnome-shell theme .css file.
 query - stdout from "update-alternatives --query gdm3.css" store in a list
 link - gdm3.css path
 best - gdm3.css alternative path selected by auto mode 
 value - current gdm3.css alternative path 
 status - whether gdm3.css is selected by manual or automatic mode 
 max - maximum Priority value of all the installed gdm3.css alternatives

 Methods:
 exist() - determines whether "mytheme" is installed as a gdm3.css alternative.
 configure() - configure "mytheme" file as a gdm3.css alternative.
 '''
 def __init__( self, mytheme=None ): 
 def _get( qvalue ):
 return [ line[ line.index('/') : ] for line in self.query if qvalue in line ][0]
 self.mytheme = mytheme
 self.query = run( [ 'update-alternatives', '--query', 'gdm3.css' ],
 stdout=PIPE, encoding="utf-8" ).stdout.splitlines()
 self.link = _get( 'Link:' )
 self.best = _get( 'Best:' )
 self.value = _get( 'Value:' )
 self.status = [ line[ line.index(':')+2 : ] for line in self.query if 'Status:' in line ][0]
 self.max = max( [ int( line[ line.index(':')+1 : ] ) for line in self.query if 'Priority:' in line ] )
 #print( f'self.query = self.query' ) #For debugging
 #print( f'self.link = self.link' ) #For debugging
 #print( f'self.best = self.best' ) #For debugging
 #print( f'self.value = self.value' ) #For debugging
 #print( f'self.status= self.status' ) #For debugging
 #print( f'self.max = self.max' ) #For debugging

 def exist( self ):
 '''Method that determines whether "mytheme.css" is installed as a gdm3.css alternative. '''
 if self.mytheme == None:
 raise TypeError( '.css file was not defined.' ) 
 if not Path( self.mytheme ).exists():
 raise FileNotFoundError( f'self.mytheme does not exist.' )
 if 'css' not in mimetypes.guess_type( self.mytheme )[0] :
 raise CSSFileTypeError( f'self.mytheme is not a css file.' )
 return True in [ True for line in self.query if self.mytheme in line ]

 def configure( self ):
 '''Method to configure my theme ".css" file as a gdm3.css alternative.'''
 def _config():
 if 'auto' not in self.status:
 run( [ 'update-alternatives', '--auto', 'gdm3.css' ] ) #Ensure auto mode is used
 run( [ 'update-alternatives', '--install', self.link, 'gdm3.css', self.mytheme, str(self.max + 1) ] )
 print( f'Configured self.mytheme as gdm3.css alternative.' )

 if not self.exist():
 _config()
 elif self.value in self.mytheme:
 print( f'self.mytheme is already gdm3.css alternative.' )
 else:
 run( [ 'update-alternatives', '--remove', 'gdm3.css', self.mytheme ] )
 self.__init__( self.mytheme )
 _config()

if __name__ == '__main__':
 #mytheme = '/usr/share/gnome-shell/theme/mytheme/mytheme.css' 
 mytheme = '/usr/share/gnome-shell/theme/mytheme/mytheme.css' #Change this to your theme
 gdm3 = GDM3_alternatives( mytheme )
 gdm3.configure()

## This script needs to be executed by sudo. ## 

How to use this script:

1. Save it to a file with .py extension, e.g. myscript.py.


2. Change line mytheme =
'/usr/share/gnome-shell/theme/mytheme/mytheme.css' to show your
   theme's .css path.


3. Run the script on terminal with sudo permission, i.e. type sudo python3.6 myscript.py. Or open your python IDLE using sudo permission and run this python script.

The version of the update-alternatives on my Ubuntu 18.04 system is:

$ update-alternatives --version
Debian update-alternatives version 1.19.0.5.

This is free software; see the GNU General Public License version 2 or
later for copying conditions. There is NO warranty.