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list, tuple, and set are all classes, yet we never created any of these collections using the class name as a function to pass in arguments to __init__, as we saw back with our Item class back in Lesson 5:

item = Item(1, "car", 2)

Well, turns out that you can treat those three collections classes the same way we treated our Item class. All of the __init__ methods accept a collection object, whether it be a list, tuple, or set:

s = {1, 2, 3}
l = list(s)
print(l)

This will output:

[1, 2, 3]

Similarly, we can convert list to set:

l = [1, 2, 3, 1]
s = set(l)
print(s)

This will output:

{1, 2, 3}

Notice how the duplicate 1 at the end is omitted in this set. Conversions between list and tuple as well as set and tuple behave similarly.

It is this ability to convert between list, set, and tuple that explains the title of this lesson. Note though that using the class syntax that we see is only good for converting. If you want to create an actual list for example from scratch, you should use the syntax described in the previous lessons, which for list you might recall uses brackets.

Finally, it should be noted that although these three classes differ in the details, there are some things in common amongst all three. A very important one is that their length can be computed using Python’s len function:

t = (1, 2, 3)
print(len(t))

This will output:

3

The len function accepts any collection. Thus, this would not work with an integer, but it will work with list, set, and tuple. In fact, it will also work with string because that is also a collection (of characters).

With that caveat explained, let’s practice what we have learned:

  • Create a tuple with the elements 5, "dog", and 5. Then convert that tuple to a set. (solution)

  • Output the length of the tuple and the set. (solution)

  • Create a list with the elements 5, "dog", and 5. Then convert that list to a tuple. (solution)

If you are feeling good about collection conversions, proceed to the next lesson!