gistfile1.py

#!/usr/bin/python3
import subprocess
import re

#repl = {6:2, 7:2} # Mouse buttons to be replaced
repl = {6:8, 7:9} # Mouse buttons to be replaced

out = subprocess.check_output(['xinput', 'list'])
for s in out.decode('utf-8').lower().split('\n'):
    try:
        s.index('mouse')
        s.index('pointer')
        r = re.search('id=([0-9]+)', s)
        if r:
            dev = r.group(1)
            buttons = subprocess.check_output(('xinput', 'get-button-map', dev)).split()

            def replacer(x):
                try: return str(repl[int(x)])
                except: return x
            buttons = list(map(replacer, buttons))

            subprocess.call(['xinput', 'set-button-map', dev]+buttons)
#        break
    except ValueError: # One of the `index` calls failed -> not a mouse pointer
        pass # Just move on, this is not an error
#else:
#    pass # Could not find an appropriate device