*args & **kwargs

*args collects any number of positional arguments into a tuple. **kwargs collects keyword arguments into a dict:

def show(*args, **kwargs):
    print(args)     # (1, 2)
    print(kwargs)   # {'a': 3}
show(1, 2, a=3)

Inside a function, loop over args like any tuple. For keyword arguments, kwargs.items() gives key/value pairs in the caller's keyword order; kwargs.values() gives just the values.

An expression such as sum(len(group) for group in groups) uses a generator expression: it supplies one calculated value at a time. Likewise, ", ".join(f"{k}={v}" for k, v in kwargs.items()) joins formatted strings with a comma and space; no items gives an empty string.

See it run

def pack(*args, **kwargs):
    count = len(args)
    return count, kwargs

n, opts = pack(7, 8, 9, color="red")
print(n, opts)

Watch what the pack() box collects from one call.

Practice *args & **kwargs

Exercise 1

total(*nums) returns the sum of all numbers given (total() is 0).
describe(**info) returns "key=value" pairs joined by ", ". For example, describe(name="Ada", lang="Py") gives "name=Ada, lang=Py".

Exercise 2

Define pack_count(*groups) to return the total number of items across any number of supplied lists; no groups gives 0. For example, pack_count([1, 2], [], [3]) is 3. It must stay fast with up to 20,000 groups.

Exercise 3

Define invoice(*amounts, **charges) to return the sum of all amounts plus all named charges, with no arguments returning 0. Define charge_names(**charges) to return the sorted charge names, including names whose charge is zero.

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Practice

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Memory · names → values

Variables appear here as Python creates them.

Output & plots