*args, **kwargs & unpacking calls

Flexible arguments

Lesson 2 of 20 · 10 min · Module 1: Functions, deeper

Some functions should accept any number of values: print() takes as many as you give it. Python collects extra arguments for you.

Start this lesson →

Free. No installation: you write and run the code in your browser, and it is checked automatically.

By the end, you’ll be able to…

Collect extra arguments with *args and **kwargs
Spread lists and dicts into calls
Keep the order of output predictable

The idea

*parts collects extra positional arguments into a tuple. **params collects extra keyword arguments into a dict. In a call, *items and **options do the opposite: they spread a list or dict into separate arguments.

def total(*numbers):
    return sum(numbers)

values = [1, 2, 3]
print(total(*values))

01Collecting

In def f(*parts, **params), f(1, 2, a=3) gives parts == (1, 2) and params == {"a": 3}. The names args and kwargs are only conventions; choose names that explain the data.

def show(*parts, **params):
    print(parts, params)

show(1, 2, level="hard")

Output

(1, 2) {'level': 'hard'}

02Spreading

In a call, * and ** reverse the process. make_url(base, *path, **query) passes each list item and each dict entry as a separate argument.

03Sort for stable output

Dicts keep insertion order, which depends on the caller. Sorting the keys makes URLs identical no matter how the arguments were written, which matters for caching and for tests.

Your turn

Write make_url(base, *parts, **params). Join base and every part with single slashes (strip slashes at the edges of each piece). If there are params, add ?key=value pairs joined by &, with keys in alphabetical order.

What the checks look for

Joins the path pieces
Removes duplicate slashes
Adds sorted query parameters
Works with only a base, and with unpacked lists and dicts

Try it in the editor →

Take this with you

Star collects in a definition and spreads in a call.

Go a little further

URL-encode the query values with urllib.parse.quote so spaces and symbols are safe.