The exercise looks at what happens when immutable objects are passed into a function. People without previous programming experience might assume that it's possible to swap 2 numbers using a function written like this. And people with limited programming experience in C or Java (or C-like languages) might see the example code and wonder if, when it comes to primitive types, Python is pass-by-value or pass-by-reference. The answer is that Python is neither. This exercise provides the opportunity to have a discussion about this particular part of the language.

There's a fairly good discussion of how arguments are passed into functions in Python here: https://jeffknupp.com/blog/2012/11/13/is-python-callbyvalue-or-callbyreference-neither/ and the Python docs on how to deal with the problem outlined by this exercise are here: https://docs.python.org/3/faq/programming.html#how-do-i-write-a-function-with-output-parameters-call-by-reference

Arguably this exercise introduces new concepts, however, I feel that it is an important "gotcha" to be aware of. The exercise could be seen to build on the discussion in the section titled "Ch-Ch-Ch-Changes" in 03-lists.html by showing how immutable objects behave when passed into a function. The exercise could be extended to get attendees to rewrite the "swap" function so that it will work as intended. An example answer would be:

a = 3
b = 7

def swap(a, b):
    temp = a
    a = b
    b = temp
    return a, b

a, b = swap(a, b)

print(a, b)