Python Function Parameter Kinds

Intermediate

Python functions can control how callers provide arguments. Beyond ordinary positional and keyword arguments, a function can require some arguments by position, require others by name, or accept a variable number of positional or keyword arguments.

Positional-only parameters

A slash in a function definition marks the parameters before it as positional-only. Callers must provide those arguments by position.

def divide(total, count, /):
    return total / count

print(divide(12, 3))

Keyword-only parameters

A bare asterisk marks the parameters after it as keyword-only. This makes calls explicit when an option is easier to understand by name.

def connect(host, *, timeout=5):
    return f"{host} (timeout={timeout})"

print(connect("example.com"))
print(connect("example.com", timeout=10))

Variable positional arguments

A parameter written with one asterisk collects remaining positional arguments into a tuple. The name is only a convention.

def total(*numbers):
    return sum(numbers)

print(total(2, 4, 6))
print(total())

Variable keyword arguments

A parameter written with two asterisks collects remaining keyword arguments into a dictionary. The name kwargs is only a convention.

def describe(**details):
    for name, value in details.items():
        print(f"{name}: {value}")

describe(language="Python", level="core")

Combining parameter kinds

A function can combine positional-only parameters, ordinary parameters, keyword-only parameters, and variable arguments. Python constrains the order so the signature remains unambiguous.

def report(name, /, *values, unit="items", **options):
    print(name, values, unit, options)

report("sales", 10, 20, unit="rows", year=2026)

Why parameter kinds matter

Parameter kinds are part of a function API contract. Use them when they make the intended calling style clearer or protect an interface from accidental dependence on parameter names.

def format_price(amount, /, *, currency="USD", places=2):
    return f"{amount:.{places}f} {currency}"

print(format_price(12.5))
print(format_price(12.5, currency="EUR", places=0))