password proxy

password_proxy.py

import argparse
import getpass

def password_proxy(value):
    password_proxy.prompt = 'Default password prompt:'
    
    class _password_proxy(str):
        def __new__(cls, value):
            obj = str.__new__(cls, value)
            obj._context = password_proxy
            return obj
        
        def __str__(self):
            if self.isempty():
                if hasattr(self._context, 'prompt'):
                    self = getpass.getpass(self._context.prompt)
                else:
                    self = getpass.getpass()
                return self
            
            def isempty(self):
                return self == ''
            
    return _password_proxy(value)

def main():
    parser = argparse.ArgumentParser()
    
    parser.add_argument('--user', default='user')
    parser.add_argument('--password', type=password_proxy, defualt='')
    
    args = parser.parse_args()
    
    print 'look ma, no password prompt (yet)'
    
    password_proxy.prompt = 'Hello {} please enter your password:'.format(args.user)
    
    print 'The password is {}'.format(args.password)
    
if __name__ == '__main__':
    main()

Try running with
python password_prompt.py --password=abcd
and
python password_prompt.py --user=bob

The first won't prompt for a password and the second will prompt for a password after printing 'look ma, no password prompt (yet)' Also notable is the password prompt can be changed based on the argparse parsed parameters.