Variable keyword arguments are very similar to variable positional arguments. The only difference is the syntax (** instead of *) and that they are collected in a dictionary. Collection and unpacking work in the same way, so let's look at an example:

# arguments.variable.keyword.py
def func(**kwargs):
    print(kwargs)

# All calls equivalent. They print: {'a': 1, 'b': 42}
func(a=1, b=42)
func(**{'a': 1, 'b': 42})
func(**dict(a=1, b=42))

All the calls are equivalent in the preceding example. You can see that adding a ** in front of the parameter name in the function definition tells Python to use that name to collect a variable number of keyword parameters. On the other hand, when we call the function, we can either pass name=value arguments explicitly, or unpack a dictionary using the same ** syntax.

The reason why being able to pass a variable number of keyword parameters is so important may not be evident at the moment, so, how about a more realistic example? Let's define a function that connects to a database. We want to connect to a default database by simply calling this function with no parameters. We also want to connect to any other database by passing the function the appropriate arguments. Before you read on, try to spend a couple of minutes figuring out a solution by yourself:

# arguments.variable.db.py
def connect(**options):
    conn_params = {
        'host': options.get('host', '127.0.0.1'),
        'port': options.get('port', 5432),
        'user': options.get('user', ''),
        'pwd': options.get('pwd', ''),
    }
    print(conn_params)
    # we then connect to the db (commented out)
    # db.connect(**conn_params)

connect()
connect(host='127.0.0.42', port=5433)
connect(port=5431, user='fab', pwd='gandalf')

Note that in the function, we can prepare a dictionary of connection parameters (conn_params) using default values as fallbacks, allowing them to be overwritten if they are provided in the function call. There are better ways to do this with fewer lines of code, but we're not concerned with that right now. Running the preceding code yields the following result:

$ python arguments.variable.db.py
{'host': '127.0.0.1', 'port': 5432, 'user': '', 'pwd': ''}
{'host': '127.0.0.42', 'port': 5433, 'user': '', 'pwd': ''}
{'host': '127.0.0.1', 'port': 5431, 'user': 'fab', 'pwd': 'gandalf'}

Note the correspondence between the function calls and the output. Notice how default values are overridden according to what was passed to the function.