19.9: Gathering keyword args
- Page ID
- 42493
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.