An overview of some neat peculiarities of the positional-only and keyword-only parameters in Python.

Positional-only parameters

The main and the most obvious feature of the positional-only parameters is that unlike regular positional parameters, their values cannot be passed as named arguments.

def add(x, /, y):
    return x + y

>>> add(1, 2)
3
>>> add(x=1, y=2)
...
TypeError: add() got some positional-only arguments passed as keyword arguments: 'x'

Unlike regular positional parameters, positional-only parameters can have a default value, in which case they are not required.

def add(x, y=0, /):
    return x + y

>>> add(1, 2)
3
>>> add(1)
1

Note that you still must observe the basic syntactic rule requiring that in a function definition, a parameter with a default value cannot be followed by a parameter without a default value.

>> def add(x=0, y, /):
...     return x + y
... 
SyntaxError: non-default argument follows default argument

Keyword-only parameters

The main and the most obvious feature of the keyword-only parameters is that unlike regular keyword parameters, their values cannot be passed as positional arguments.

def add(x, *, y=0):
    return x + y

>>> add(1)
1
>>> add(1, 2)
...
TypeError: add() takes 1 positional argument but 2 were given

Unlike regular keyword parameters, keyword-only parameters don’t need to have a default value, in which case they are required.

def add(x, *, y):
    return x + y

>>> add(1, y=2)
3
>>> add(1)
...
TypeError: add() missing 1 required keyword-only argument: 'y'

Interestingly, within the group of keyword-only parameters in a function definition, a parameter with a default value can even be followed by a parameter without a default value.

def add(x, *, y=0, z):
    return x + y + z

>>> add(1, y=2, z=3)
6
>>> add(1, z=3)
4