Example of using variable length keyword arguments and unpacking arguments

Using variable length arguments and unpacking arguments you can pass arguments between functions. Let me give you an example.

Function check_passwd (func_add_user_kwargs_example.py file):

In [1]: def check_passwd(username, password, min_length=8, check_username=True):
   ...:     if len(password) < min_length:
   ...:         print('Password is too short')
   ...:         return False
   ...:     elif check_username and username in password:
   ...:         print('Password contains username')
   ...:         return False
   ...:     else:
   ...:         print(f'Password for user {username} has passed all checks')
   ...:         return True
   ...:

Function checks password and returns True if password has passed verification and False if not.

Call function in ipython:

In [3]: check_passwd('nata', '12345', min_length=3)
Password for user nata has passed all checks
Out[3]: True

In [4]: check_passwd('nata', '12345nata', min_length=3)
Password contains username
Out[4]: False

In [5]: check_passwd('nata', '12345nata', min_length=3, check_username=False)
Password for user nata has passed all checks
Out[5]: True

In [6]: check_passwd('nata', '12345nata', min_length=3, check_username=True)
Password contains username
Out[6]: False

We will create add_user_to_users_file function that requests password for specified user, checks it and requests it again if password has not been checked or writes user and password to file if password has been verified

In [7]: def add_user_to_users_file(user, users_filename='users.txt'):
   ...:     while True:
   ...:         passwd = input(f'Enter password for user {user}: ')
   ...:         if check_passwd(user, passwd):
   ...:             break
   ...:     with open(users_filename, 'a') as f:
   ...:         f.write(f'{user},{passwd}\n')
   ...:

In [8]: add_user_to_users_file('nata')
Enter password for user nata: natasda
Password is too short
Enter password for user nata: natasdlajsl;fjd
Password contains username
Enter password for user nata: salkfdjsalkdjfsal;dfj
Password for user nata has passed all checks

In [9]: cat users.txt
nata,salkfdjsalkdjfsal;dfj

In this version of add_user_to_users_file() function, it is not possible to regulate the minimum password length and whether to verify the presence of a username in password. In the following version of add_user_to_users_file() function, these features are added:

In [5]: def add_user_to_users_file(user, users_filename='users.txt', min_length=8, check_username=True):
   ...:     while True:
   ...:         passwd = input(f'Enter password for user {user}: ')
   ...:         if check_passwd(user, passwd, min_length, check_username):
   ...:             break
   ...:     with open(users_filename, 'a') as f:
   ...:         f.write(f'{user},{passwd}\n')
   ...:

In [6]: add_user_to_users_file('nata', min_length=5)
Enter password for user nata: natas2342
Password contains username
Enter password for user nata: dlfjgkd
Password for user nata has passed all checks

You can now specify min_length or check_username when calling a function. However, it was necessary to repeat parameters of check_passwd function in defining of add_user_to_users_file function. This is not very good and when there are many parameters it is just inconvenient, especially considering that check_passwd function can have other parameters.

This happens quite often and Python has a common solution to this problem: all arguments for internal function (in this case it is check_passwd) will be taken in **kwargs. Then, when calling check_passwd() function they will be unpacked into keyword arguments by the same **kwargs syntax.

In [7]: def add_user_to_users_file(user, users_filename='users.txt', **kwargs):
   ...:     while True:
   ...:         passwd = input(f'Enter password for user {user}: ')
   ...:         if check_passwd(user, passwd, **kwargs):
   ...:             break
   ...:     with open(users_filename, 'a') as f:
   ...:         f.write(f'{user},{passwd}\n')
   ...:

In [8]: add_user_to_users_file('nata', min_length=5)
Enter password for user nata: alskfdjlksadjf
Password for user nata has passed all checks

In [9]: add_user_to_users_file('nata', min_length=5)
Enter password for user nata: 345
Password is too short
Enter password for user nata: 309487538
Password for user nata has passed all checks

In this version you can add arguments to check_passwd() function without having to duplicate them in add_user_to_users_file function.