When writing a function with default arguments, it can be tempting to include an empty list as a default argument to that function. Let’s say you have a function called populate_list that has two required arguments, but it’s easy to see that we might want to give it some default arguments in case we don’t have either list_to_populate or length every time. So we’d give it these defaults:

def populate_list(list_to_populate=[], length=1): for num in range(length): list_to_populate.append(num) return list_to_populate

It’s reasonable to believe that list_to_populate will be given an empty list every time it’s called. This isn’t the case! list_to_populate will be given a new list once, in its definition, and all subsequent function calls will modify the same list. This will happen:

returned_list = populate_list(length=4) print(returned_list) # Prints [0, 1, 2, 3] -- this is expected returned_list = populate_list(length=6) print(returned_list) # Prints [0, 1, 2, 3, 0, 1, 2, 3, 4, 5] -- this is a surprise!

When we call populate_list a second time we’d expect the list [0, 1, 2, 3, 4, 5]. But the same list is used both times the function is called, causing some side-effects from the first function call to creep into the second. This is because a list is a mutable object.

A mutable object refers to various data structures in Python that are intended to be mutated, or changed. A list has append and remove operations that change the nature of a list. Sets and dictionaries are two other mutable objects in Python.

It might be helpful to note some of the objects in Python that are not mutable (and therefore OK to use as default arguments). int, float, and other numbers can’t be mutated (arithmetic operations will return a new number). tuples are a kind of immutable list. Strings are also immutable — operations that update a string will all return a completely new string.



In script.py we’ve written a helper function that adds a new menu item to an order in a point-of-sale system. As you can see, we can start a new order by calling update_order without an argument for current_order. Unfortunately, there’s a bug in our code causing some previous order contents to show up on other people’s bills!

First, try to guess what the output of this code will be. Then, run script.py.

We’ll fix this function in the next exercise, if you want more of an explanation of what’s happening here, check out the hint!

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