Keyword-Only and Positional-Only Arguments

This text is part of a Real Python tutorial by John Sturtz.

Keyword-Only Arguments

A Python function in version 3.x can be defined so that it takes keyword-only arguments. These are function arguments that must be specified by keyword. Let’s explore a situation where this might be beneficial.

Suppose you want to write a Python function that takes a variable number of string arguments, concatenates them together separated by a dot ("."), and prints them to the console. Something like this will do to start:

>>> def concat(*args):
...     print(f'-> {".".join(args)}')
...

>>> concat('a', 'b', 'c')
-> a.b.c
>>> concat('foo', 'bar', 'baz', 'qux')
-> foo.bar.baz.qux

As it stands, the output prefix is hard-coded to the string '-> '. What if you want to modify the function to accept this as an argument as well, so the user can specify something else? This is one possibility:

>>> def concat(prefix, *args):
...     print(f'{prefix}{".".join(args)}')
...

>>> concat('//', 'a', 'b', 'c')
//a.b.c
>>> concat('... ', 'foo', 'bar', 'baz', 'qux')
... foo.bar.baz.qux

This works as advertised, but there are a couple of undesirable things about this solution:

1. The prefix string is lumped together with the strings to be concatenated. Just from looking at the function call, it isn’t clear that the first argument is treated differently from the rest. To know that, you’d have to go back and look at the function definition.

2. prefix isn’t optional. It always has to be included, and there’s no way to assume a default value.

You might think you could overcome the second issue by specifying a parameter with a default value, like this, perhaps:

>>> def concat(prefix='-> ', *args):
...     print(f'{prefix}{".".join(args)}')
...

Unfortunately, this doesn’t work quite right. prefix is a positional parameter, so the interpreter assumes that the first argument specified in the function call is the intended output prefix. This means there isn’t any way to omit it and obtain the default value: