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Functions

Parameters and Arguments

Positional, keyword, default, and keyword-only arguments

Interview: Critical for interviews — mutable defaults is one of the most asked Python questions

Last Updated: June 12, 2026 • 11 min read

Python provides an incredibly flexible parameter system. Understanding the different parameter types, their ordering rules, and how arguments are matched to parameters is essential for writing clean, Pythonic functions and is a frequent interview topic.

Parameter Types

  • Positional parameters: Required, matched by position — def func(a, b)
  • Default parameters: Optional with default values — def func(a, b=10)
  • Keyword-only parameters: Must be passed by name (after *) — def func(a, *, b)
  • Positional-only parameters: Cannot be passed by keyword (before /, Python 3.8+) — def func(a, /, b)
  • args: Collects extra positional arguments into a tuple
  • *kwargs: Collects extra keyword arguments into a dict

Parameter Ordering Rules

Parameters must follow this exact order:

  1. Positional-only (before /)
  2. Regular positional/keyword
  3. *args
  4. Keyword-only (between *args and **kwargs)
  5. **kwargs

The Mutable Default Argument Trap

This is the #1 Python interview question about parameters. Default values are evaluated once at function definition time, not at each call. Using def func(lst=[]) means ALL calls share the same list. Always use def func(lst=None) and create the mutable object inside the function.

Positional-Only and Keyword-Only

  • Positional-only (/): Forces callers to pass args by position — used in built-in functions like len(obj, /) to prevent keyword usage
  • Keyword-only (): Forces callers to use named args — improves readability for boolean flags and configuration options
  • Bare : You can use * without a name to separate positional from keyword-only parameters

Argument Unpacking

  • operator: Unpacks a list/tuple into positional arguments — func(*[1, 2, 3])
  • * operator: Unpacks a dict into keyword arguments — func(**{"a": 1, "b": 2})
  • Combined: func(*args, **kwargs) forwards all arguments to another function

Pro Tip: Force Keyword Arguments

Use * to force callers to use keyword arguments for clarity: def connect(host, port, *, timeout=30, retries=3). This prevents connect("localhost", 5432, 60, 5) where it's unclear what 60 and 5 mean.

Use Cases

Designing flexible APIs with optional and keyword-only parameters

Building decorator wrappers that forward arbitrary arguments with `*args`, `**kwargs`

Creating configuration functions where keyword-only args improve readability

Implementing factory functions with sensible defaults

Writing generic wrapper functions in frameworks and middleware

Common Mistakes

Using mutable objects (list, dict) as default parameter values — shared across calls

Putting default parameters before non-default parameters — causes SyntaxError

Forgetting the ordering rule: positional-only → normal → `*args` → keyword-only → `**kwargs`

Not using keyword-only (*) for boolean flags — makes calls like func("x", True, False) unreadable

Confusing `*args` (collects into tuple) with `**kwargs` (collects into dict)