This article explains what *args means in Python and how to use it in your own functions.

What is *args and how do you use it?

You probably know the built-in function max. What does it do? You give it some values, and it returns the largest one.

How many values can you give it? However many you want! Here are three examples below:

>>> max(1, 2)
2
>>> max(1, 2, 3)
3
>>> max(1, 2, 3, 4, 5, 6, 7)
7

The number of arguments that the function max accepts is variable.

It's not just 2. It's not just 2 or 3. It's not just 2, 3, or 7. It's any number of arguments. You could even pass it 100 or 1000 arguments.

But now, think about how you would implement a function that works like this. How do you create a function that takes an arbitrary number of arguments?

For example, how do you take the function add below and make it so that it works with as many arguments as you want?

def add(a, b):
    return a + b

Right now, it only accepts 2 arguments. What if you want it to accept 1, 2, 7, 100, or 1000 arguments? That's when *args comes in.

If you use *args as a parameter, Python knows that it can collect as many arguments as the user passes in, and they will be stored inside the tuple args.

Here's a function that prints its args so that we can see what args really is:

>>> def print_args(*args):
...     print(f"args is a {type(args)} with {len(args)} elements.")
...     print(args)
...

>>> print_args(1)
args is a <class 'tuple'> with 1 elements.
(1,)

>>> print_args(1, 2)
args is a <class 'tuple'> with 2 elements.
(1, 2)

>>> print_args([73, 42])
args is a <class 'tuple'> with 1 elements.
([73, 42],)

>>> print_args([73, 42], "oi", "rodrigo", False)
args is a <class 'tuple'> with 4 elements.
([73, 42], 'oi', 'rodrigo', False)

>>> print_args() args is a <class 'tuple'> with 0 elements.
()

There's one thing I want you to understand, now. The 5 characters *args aren't special. What matters is the asterisk * immediately to the left of a parameter name. Typically, in Python we use *args, but you could've typed *bananas or *abracadabra.

This new version of print_args below works just as well:

def print_args(*abracadabra):
    print(f"abracadabra is a {type(abracadabra)} with {len(abracadabra)} elements.")
    print(abracadabra)

Here's a summary of what we've seen so far:

  • *args enables functions to receive an arbitrary number of arguments;
  • it's the asterisk * on the left of a parameter name that makes this possible, we just use *args as a convention; and
  • args is always a tuple, which can be empty.

Now, with all this knowledge, you can implement the function add as I challenged you above. How would you do it? Something like this:

>>> def add(*numbers_to_add):
...     added = 0
...     for number in numbers_to_add:
...         added += number
...     return added
...
>>> add(1, 2, 3, 4, 5)
15

Even better would be to just leverage the built-in sum, given that numbers_to_add is a tuple:

# Or even better, given that “numbers_to_add” is a tuple:
>>> def add(*numbers_to_add):
...     return sum(numbers_to_add)
...
>>> add(1, 2, 3, 4, 5)
15

So, this is the magic of *args.

We're just scratching the surface, though. The asterisk * can be used in Python for many more (related) things. For example, you can read about unpacking with starred assignments.

Take your Python 🐍 skills to the next level 🚀

I write about Python every week. Join +16.000 others who are taking their Python 🐍 skills to the next level 🚀, one email at a time.

References

Previous Post Next Post

Blog Comments powered by Disqus.