Named arguments with defaults with keyword arguments and **kw#

Python supports named keyword arguments. For example, the list.sort method takes an optional key argument, specifying how to perform the sort. Here’s an example:

The syntax for calling a function with keywords is to write f(arg1, arg2, ..., argn, kw1=val1, k2=val2, ...).

Defining keyword arguments#

To define a function that takes keyword arguments, you write def f(arg1, ..., argn, kw1=val1, ...). Here’s an example to play with:

Here’s an example interaction:

>>> f()
kw1 None kw2 None
>>> f(kw2=True)
kw1 None kw2 True
>>> f(kw1=True)
kw1 True kw2 None
>>> f(1)
kw1 1 kw2 None
>>> f(1, 2)
kw1 1 kw2 2
>>> f(1,kw2=2)
kw1 1 kw2 2
>>> f(1,kw1=2)
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
TypeError: f() got multiple values for argument 'kw1'
>>> f(kw2=True, 2)
    File "<stdin>", line 1
        f(kw2=True, 2)
SyntaxError: positional argument follows keyword argument

The first example shows that keyword arguments take their default values. The second and third examples show that you can specify only one keyword argument, and the others will take their defaults. The fourth and fifth examples show that keyword arguments can be treated as positional arguments: when we write f(1), Python sees that there are no declared positional arguments, so it starts filling in the keywords from left to right. The sixth example shows that you can mix the two, but the seventh shows that you can’t write positional and keyword arguments for the same keyword. Finally, the eighth example shows that you can’t give positional arguments after keyword arguments.

Arbitrary keyword arguments with **kw#

Just as *args lets you define variable arity, the **kw form lets you capture all keyword arguments. Like *args, you can use the ** form both when defining and when calling a function. Here’s an example definition:

And here’s an example interaction:

None and {}
>>> g(kw1=True)
True and {}
>>> g(kw2=False)
None and {'kw2': False}
>>> g(**{'a': 3, 'b': 4, 'kw1': 5})
5 and {'a': 3, 'b': 4}
>>> g(kw1=1, **{'kw1': 2})
Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
TypeError: __main__.g() got multiple values for keyword argument 'kw1'

We’ve defined g with both an explicit keyword argument as well as **kw. In general, that’s a terrible idea! You should either use **kw or explicit keyword arguments, but probably not both.

Dangerous defaults#

The default value you provide for a keyword is computed once. Here’s an example of how it can go wrong:

What?! Why is are the 3, 2, and 1 there in the second call? There’s only one list for acc’s default value. Each time we call insert, we update that list—and all future calls will see it.

There are two solutions:

  1. Never mutate a keyword argument. Rather than calling acc.insert, we could write acc = [x] + acc to create a new list.

  2. Use None as the default, and test at the front to initialize with the mutable value.

Here’s an example of both: