Python3.8 - Position-Only Parameters

Published on:
March 11, 2022

Let's discuss something very interesting and useful improvement introduced in python3.8 (PEP 570), the position-only parameters, This is a function parameter syntax `/ ` to indicate that some function parameters must be specified positionally and cannot be used as keyword arguments.


In above function parameter are explained as:

pos1, pos2
- position-only parameters

/ - tells us that the parameters appearing before this sign are position-only.
pos_or_kw1, pos_or_kw2 - position-or-keyword parameters

* - tells us that the parameters appearing before this sign are of position-or-keyword type.
kw1, kw2
- keyword-only parameters

  • With positional-or-keyword parameters, the mix of calling conventions is not always desirable. Addition of / marker for positional-only arguments improves the language’s consistency.
  • For the cases where sequence of parameters to the function matters a lot and can not be compromised.
  • There are some cases where parameter names does not really matter and provide no true meaning.

             e.g. min(arg1, arg2)

  • Python library authors would have the flexibility to change the name of positional-only parameters without breaking callers.

Note: Do not use position-only parameters when parameter names have meaning and the function definition is more understandable by being explicit with names, go with keyword-only parameters instead.

   e.g. namedtuple(typenames, field_names, …)

How to use position-only keywords:

different functions calls for `pow` would be like below style:

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