Python Argument & Unpacking Tricks — A Practical Tutorial

This tutorial covers the most useful Python argument and unpacking patterns: *, **, *args, **kwargs, keyword-only, positional-only, and common idioms.

1. Positional vs. Keyword Arguments

def f(a, b):
    print(a, b)

f(1, 2)
f(a=1, b=2)
f(1, b=2)

positional
keyword
mixed

In def f(a, b), parameters are positional-or-keyword by default.

2. * — Unpack a Sequence into Positional Arguments

def f(a, b):
    print(a, b)

lst = [1, 2]
f(*lst)

Equivalent to f(1, 2)

Rule: *sequence → positional arguments.

3. ** — Unpack a Dict into Keyword Arguments

def f(a, b):
    print(a, b)

d = {"a": 1, "b": 2}
f(**d)

Equivalent to f(a=1, b=2)

Rule: **dict → keyword arguments. Keys must match parameter names.

4. *args — Collect Extra Positional Arguments

def f(*args):
    print(args)

f(1, 2, 3)

(1, 2, 3)

Use cases: unknown number of inputs, wrappers/decorators, forwarding arguments.

5. **kwargs — Collect Extra Keyword Arguments

def f(**kwargs):
    print(kwargs)

f(a=1, b=2)

{'a': 1, 'b': 2}

Use cases: configuration objects, flexible APIs, passing JSON-like data.

6. Forwarding Arguments

def wrapper(*args, **kwargs):
    return real_function(*args, **kwargs)

Common in decorators, FastAPI routing, Pydantic models, and tool orchestration.

7. * in Function Definitions — Keyword-Only Arguments

def f(a, *, b):
    print(a, b)

f(1, b=2)

OK

f(1, 2)

TypeError

Use when you want clarity or stable public APIs.

8. / in Function Definitions — Positional-Only Arguments

def f(a, b, /):
    print(a, b)

f(1, 2)

OK

f(a=1, b=2)

TypeError

Common in built-ins and performance-critical APIs.

9. Extended Unpacking (Destructuring)

a, *middle, b = [1, 2, 3, 4, 5]
print(a, middle, b)

1 [2, 3, 4] 5

Useful for flexible slicing and sequence parsing.

10. * vs. ** — Common Mistakes

*[1, 2]        # OK
**[1, 2]       # TypeError (list is not a mapping)

*{"a": 1}     # 'a'
**{"a": 1}    # a=1

11. Why Pydantic and FastAPI Rely on This

data = {"text": "hello", "threshold": "0.7"}
Params(**data)

Models expect named fields, so **data turns a dict into keyword arguments and triggers validation automatically.

12. Mental Model

13. One-Sentence Takeaway

Python’s * and ** are the glue that connects lists, dictionaries, JSON, APIs, and function calls.