Argument order rules in Python

Introduction

Arguments in a function must be given in the right order so Python knows which value goes to which parameter.

When you write a function that takes multiple inputs.

When you call a function and want to give values to its parameters.

When you want to avoid mistakes by mixing up inputs.

When you use both positional and keyword arguments in a function call.

When you want to use default values but still provide some arguments.

Syntax

Python

def function_name(positional1, positional2, /, positional_or_keyword1, positional_or_keyword2, *, keyword_only1, keyword_only2):
    pass

Positional-only arguments come first and must be given by position.

Keyword-only arguments come after * and must be given by name.

Examples

name is positional-only, greeting can be positional or keyword, punctuation is keyword-only.

Python

def greet(name, /, greeting='Hello', *, punctuation='!'):
    print(f"{greeting}, {name}{punctuation}")

Shows different ways to call the function following argument order rules.

Python

greet('Alice')
greet('Bob', 'Hi')
greet('Carol', greeting='Hey', punctuation='.')

Sample Program

This program shows how to use positional-only, positional-or-keyword, and keyword-only arguments in a function.

Python

def describe_pet(name, /, species='dog', *, age=None):
    description = f"Pet name: {name}, species: {species}"
    if age is not None:
        description += f", age: {age} years"
    print(description)

describe_pet('Buddy')
describe_pet('Mittens', 'cat')
describe_pet('Goldie', age=2)

Important Notes

Positional-only arguments are marked with a slash / in the function definition.

Keyword-only arguments come after an asterisk * and must be named when calling the function.

Using argument order rules helps avoid confusion and bugs in your code.

Summary

Arguments have a specific order: positional-only, positional-or-keyword, then keyword-only.

Positional-only arguments must be given by position, keyword-only must be named.

Following these rules makes your functions easier to use and understand.