def loadclass():
    import sys, importlib
    modulename = sys.argv[1]                          # Module name in command line
    module = importlib.import_module(modulename)      # Import module by name string
    print(f'[Using: {module.CardHolder}]')            # No need for getattr() here
    return module.CardHolder

def printholder(who):
    print(who.acct, who.name, who.age, who.remain, who.addr, sep=' / ')

if __name__ == '__main__':
    CardHolder = loadclass()
    bob = CardHolder('1234-5678', 'Bob Smith', 40, '123 main st')
    printholder(bob)
    bob.name = 'Bob Q. Smith'
    bob.age  = 50
    bob.acct = '23-45-67-89'
    printholder(bob)

    sue = CardHolder('5678-12-34', 'Sue Jones', 35, '124 main st')
    printholder(sue)
    try:
        sue.age = 200
    except: print('Bad age for Sue')

    try:
        sue.remain = 5
    except: print("Can't set sue.remain")

    try:
        sue.acct = '1234567'
    except: print('Bad acct for Sue')