*args and **kwargs

Python · Reference cheat sheet

📋 Overview

*args collects extra positional arguments into a tuple; **kwargs collects extra keyword arguments into a dict. Use them for wrappers, decorators, forward-compatible APIs, and when calling through to another function. Prefer explicit parameters when the API is stable and small.

🔧 Core concepts

Names args/kwargs are convention only; *rest / **options are fine.

💡 Examples

Collect and forward:

def logged(fn):
    def wrapper(*args, **kwargs):
        print(f"call {fn.__name__}")
        return fn(*args, **kwargs)
    return wrapper

@logged
def add(a: int, b: int) -> int:
    return a + b

add(1, 2)

Mix with explicit params:

def connect(host: str, port: int = 5432, /, *, timeout: float = 5.0, **opts):
    print(host, port, timeout, opts)

connect("localhost", 5432, timeout=2.0, ssl=True)

Unpacking into calls:

def greet(name: str, excited: bool = False) -> str:
    return f"Hi {name}{'!' if excited else '.'}"

args = ("Ada",)
kwargs = {"excited": True}
print(greet(*args, **kwargs))

Merge kwargs safely:

def create_user(name: str, **kwargs):
    defaults = {"active": True, "role": "user"}
    data = {**defaults, **kwargs, "name": name}
    return data

⚠️ Pitfalls

• Overusing **kwargs hides required fields and breaks autocomplete/type checkers—annotate with TypedDict or explicit params when possible.
• Duplicate keywords: f(1, a=1, **\{"a": 2\}) raises TypeError.
• Mutating the received kwargs dict can surprise callers if you store it; copy when retaining.
• *args typing is limited; prefer *args: int (homogeneous) or redesign.
• Forgetting to forward *args, **kwargs in wrappers drops arguments silently until call time.

🔗 Related

• Types
• Dictionaries
• Docstrings
• Import / export
• Loops