How to create a python function which take only positional arguments and no keyword arguments?

Question

A lot of inbuilt functions in python don't take keyword arguments. For example, the chr function.

>>> help(chr)
Help on built-in function chr in module builtins:

chr(i, /)
    Return a Unicode string of one character with ordinal i; 0 <= i <= 0x10ffff.

Trying to pass values to chr using keyword arguments don't work.

>>> chr(i=65)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: chr() takes no keyword arguments

I know that the / character in the help text of the chr function means that it won't take keyword arguments.

How can I define a function that does not take keyword arguments? And of course, I want to define a function that takes arguments, but only positional arguments.

This will probably be marked as a duplicate but at least that way I'll get the answer. I can't find a StackOverflow answer for this question.

Another similar feature I learnt is to create a function that does not take positional arguments.

>>> def f(*, a, b):
...     print(a, b)
...

>>> f(1, 2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() takes 0 positional arguments but 2 were given

>>> f(a=1, b=2)
1 2

This question is similar to mine, but it doesn't actually answer my question. I still don't know how to define a function that will not accept keyword arguments, like several of the built-in functions.

That was found by copy-pasting your question title onto Google. :| Although admittedly, it's catered to Python 2 and not 3. — TrebledJ
– TrebledJ, Commented Jan 18, 2019 at 10:42
… and, why would you want to do this? PEP570 seems to imply that this would allow you to use parameter names which don’t have to make much sens on the outside. — Manngo
– Manngo, Commented Jan 31, 2024 at 23:49

ForceBru · Accepted Answer · 2019-01-18 13:34:39Z

3

There's PEP 570, which is only a draft, so one cannot create positional-only arguments in pure Python. This can, however, be done in a function written in C for Python.

answered Jan 18, 2019 at 13:34

ForceBru

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3 Comments

Diptangsu Goswami Over a year ago

I suppose this feature is coming to python 3.8

ForceBru Over a year ago

@DiptangsuGoswami, since the PEP has been accepted, it sure is!

Diptangsu Goswami Over a year ago

Now that this feature is part of python, could you please edit your answer and add an example for this feature?

BlueLightning42 · Accepted Answer · 2020-11-11 04:35:19Z

Seeing as how the previous answer never got updated to 3.8 here's a brief answer for future readers

the / character in a function declaration marks all arguments before as positional only

def func(a, b, /):
    print(a ** b)

func(2, 4) # safe
func(a=2, b=4) # got some positional-only arguments passed as keyword arguments: 'a, b'

and the * character in a function declaration marks all arguments after as keyword only

def func(*, a, b):
    print(a ** b)

func(a=2, b=4) # safe
func(2, 4) # takes 0 positional arguments but 2 were given

these declarations can be combined to create a function with all three options positional only- default(both)- keyword only

def func(a, b, /, c, *, d, e):
    pass #too lazy to think of a use case

func(1, 2, 3, d=4, e=5) # safe
func(6, 7, c=8, d=9, e=10) # safe
func(1, b=2, c=3, d=4, e=5) # got some positional-only arguments passed as keyword arguments: 'b'
# etc

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