*args and **kwargs

*args

*args allows a function to accept any number of positional arguments. Instead of defining a fixed number of parameters, the single *args parameter collects all the positional arguments passed in and packages them into a tuple inside the function.

The * is what does the work. The word args is just a convention. You could call it *numbers or *items and it would work exactly the same way. But *args is what every developer expects to see so stick with it.

Without *args a function that adds numbers would need a fixed number of parameters:

This works for exactly two numbers. But what if you need to add three? Or ten? With *args the function handles any number of arguments without any changes:

Because *args is a tuple inside the function we can loop over it, pass it to other functions or use any tuple operation on it. This makes it extremely flexible.

**kwargs

**kwargs works the same way but for keyword arguments. Instead of collecting positional arguments into a tuple, it collects keyword arguments into a dictionary where each keyword becomes a key and each value becomes the corresponding value.

Again the ** is what does the work and kwargs is just a convention. You will see it written as **kwargs everywhere so keep it consistent.

Because **kwargs is a dictionary inside the function we can iterate over it using .items() just like any other dictionary. This makes it perfect for situations where you want to pass in a flexible set of named options or settings.

Using *args and **kwargs Together

You can use both in the same function. When you do, the order matters. Regular parameters come first, then *args, then **kwargs. Python will throw an error if you mix up the order.

In the example below we create a function that takes a label as a regular parameter, a set of numbers via *args and a set of additional details via **kwargs. The function adds all the numbers together and displays the result alongside the keyword details.

Exercise 1: *args Basic

• Task: Define a function called multiply_all() that uses *args to accept any number of numbers.
• Goal: Inside the function multiply all the numbers together and return the result. Call the function four times with a different number of arguments each time. Write a comment explaining what data type *args is inside the function and why that allows you to loop over it.

Exercise 2: *args with a Condition

• Task: Define a function called count_above(threshold, *args) that takes a threshold number as the first parameter and any number of additional numbers via *args.
• Goal: Inside the function count how many of the numbers in *args are greater than the threshold and return that count. Call the function with a threshold of 5 and the numbers 1, 3, 7, 9, 2, 8, 4, 6. Write a comment explaining why the threshold must come before *args in the parameter list.

Exercise 3: **kwargs Basic

• Task: Define a function called build_profile() that uses **kwargs to accept any number of keyword arguments.
• Goal: Inside the function print each key and value in the following format: name: Eric. Call the function twice with a completely different set of keyword arguments each time to demonstrate its flexibility. Write a comment explaining what data type **kwargs is inside the function.

Exercise 4: *args and **kwargs Together

• Task: Define a function called order_summary(order_id, *items, **details) that takes an order ID as a regular parameter, any number of items via *args and any number of keyword details via **kwargs.
• Goal: Inside the function print the order ID, list every item ordered and print each detail. Call the function with an order ID of 1042, items of 'pizza', 'pasta', 'tiramisu' and details of customer='Eric', table=5, paid=True. Write a comment explaining the correct order of parameters and what happens if you mix them up.

Exercise 5: Real World *args and **kwargs

• Task: Define a function called generate_report(title, *scores, **metadata).
• Goal: Inside the function use *scores to calculate the total, average, highest and lowest score. Use **metadata to print any additional information passed in. Call the function with a title of 'Q1 Results', scores of 78, 92, 55, 88, 74, 96, 61 and metadata of teacher='Ms Smith', subject='Python', term='Spring'. Add a docstring to the function explaining what each parameter does.