Python 3. Ассоциирование расширений файлов со своей программой.

import subprocess
 
with open('out.txt','w') as outfile:
    subprocess.call('reg query HKEY_CLASSES_ROOT',stdout=outfile,stderr=outfile,shell=False)

regdata=open('out.txt').read()
 
if not 'HKEY_CLASSES_ROOT\Proga' in regdata:
    main=os.getcwd()+'\\Proga.exe'
    command=r'reg add HKEY_CLASSES_ROOT\Proga\shell\open\command /ve /f /d "\"'+main+'\\" \\"%1\\""'
    with open('out.txt','w') as outfile:
        subprocess.call(command,stdout=outfile,stderr=outfile,shell=False)

with open('out.txt','w') as outfile:
    subprocess.call('reg add HKEY_CLASSES_ROOT\.html\OpenWithProgIDs /f /v Proga',stdout=outfile,stderr=outfile,shell=False)

import sys
file=sys.argv[1]