Python - Positional-Only Arguments

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Positional Only Arguments

It is possible in Python to define a function in which one or more arguments can not accept their value with keywords. Such arguments may be called positional-only arguments.

Example

Python's built-in input() function is an example of positional-only arguments. The syntax of input function is −

input(prompt = "")

Prompt is an explanatory string for the benefit of the user. For example −

name = input("enter your name ")

However, you cannot use the prompt keyword inside the parantheses.

   name = input (prompt="Enter your name ")
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: input() takes no keyword arguments

To make an argument positional-only, use the "/" symbol. All the arguments before this symbol will be treated as position-only.

Positional-Only Arguments Example

We make both the arguments of intr() function as positional-only by putting "/" at the end.

def intr(amt, rate, /):
   val = amt*rate/100
   return val

If we try to use the arguments as keywords, Python raises following error message −

   interest = intr(amt=1000, rate=10)
              ^^^^^^^^^^^^^^^^^^^^^^^
TypeError: intr() got some positional-only arguments passed as keyword arguments: 'amt, rate'

A function may be defined in such a way that it has some keyword-only and some positional-only arguments.

def myfunction(x, /, y, *, z):
   print (x, y, z)

In this function, x is a required positional-only argument, y is a regular positional argument (you can use it as keyword if you want), and z is a keyword-only argument.

The following function calls are valid −

myfunction(10, y=20, z=30)
myfunction(10, 20, z=30)

However, these calls raise errors −

   myfunction(x=10, y=20, z=30)
TypeError: myfunction() got some positional-only arguments passed as keyword arguments: 'x'

   myfunction(10, 20, 30)
TypeError: myfunction() takes 2 positional arguments but 3 were given