Functions — *args, **kwargs & Keyword-Only Args

Packing and unpacking, parameter ordering rules, keyword-only arguments, and call-site unpacking

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Explanation

*args and **kwargs — packing:** ``python def show(*args, **kwargs): print(args) # tuple of positional args print(kwargs) # dict of keyword args show(1, 2, 3, name='Alice', age=30) # (1, 2, 3) # {'name': 'Alice', 'age': 30}

Parameter ordering rule (must follow this exactly): ``python def f(positional, *args, keyword_only, **kwargs): pass # 1. Regular positional params # 2. *args (captures extra positionals) # 3. Keyword-only params (after * — MUST be passed by name) # 4. **kwargs (captures extra keywords)

Keyword-only arguments — forcing callers to be explicit: ``python def connect(host, port, *, timeout=30, retries=3): # ^ the bare * means everything after is keyword-only pass connect('localhost', 8080, timeout=5) # OK connect('localhost', 8080, 5) # TypeError — timeout must be named

Unpacking at call site:

python nums = [1, 2, 3] print(*nums) # same as print(1, 2, 3) config = {'host': 'localhost', 'port': 8080} connect(**config) # same as connect(host='localhost', port=8080)

Examples

Practical use: wrapper function

*args/**kwargs in wrappers/decorators must pass through to the original function

import time

def timer(func):
    def wrapper(*args, **kwargs):   # accepts ANYTHING the wrapped fn accepts
        start = time.time()
        result = func(*args, **kwargs)  # passes everything through
        print(f"{func.__name__} took {time.time()-start:.3f}s")
        return result
    return wrapper

@timer
def slow(n, *, label='result'):
    time.sleep(0.1)
    return f"{label}: {n}"

print(slow(42, label='answer'))

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