19.9: Gathering keyword args

In Section 12.4, we saw how to write a function that gathers its arguments into a tuple:

def printall(*args):
    print(args)

You can call this function with any number of positional arguments (that is, arguments that don’t have keywords):

>>> printall(1, 2.0, '3')
(1, 2.0, '3')

But the * operator doesn’t gather keyword arguments:

>>> printall(1, 2.0, third='3')
TypeError: printall() got an unexpected keyword argument 'third'

To gather keyword arguments, you can use the ** operator:

def printall(*args, **kwargs):
    print(args, kwargs)

You can call the keyword gathering parameter anything you want, but kwargs is a common choice. The result is a dictionary that maps keywords to values:

>>> printall(1, 2.0, third='3')
(1, 2.0) {'third': '3'}

If you have a dictionary of keywords and values, you can use the scatter operator, ** to call a function:

>>> d = dict(x=1, y=2)
>>> Point(**d)
Point(x=1, y=2)

Without the scatter operator, the function would treat d as a single positional argument, so it would assign d to x and complain because there’s nothing to assign to y:

>>> d = dict(x=1, y=2)
>>> Point(d)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __new__() missing 1 required positional argument: 'y'

When you are working with functions that have a large number of parameters, it is often useful to create and pass around dictionaries that specify frequently used options.

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