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At the end of the previous lecture, I left 
you thinking about why it's important to 

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understand that sets are unordered.
What are the implications of that, 

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when we write our code?
There are two that immediately spring to mind. 

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The first is, you can't index into a set. If 
items in a set don't have a fixed position, 

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then it just doesn't make sense to try 
to get an item at a certain position. 

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Let's demonstrate that with some code: 

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I've created a list, on line 9. On line 10, 
we retrieve the item at index position 3. 

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"goat" is the fourth animal in the list, and when 
we run the program, we get "goat" printed out. 

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I'll run the program a few times, to 
make sure we get "goat" every time. 

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Ok, I'm in danger of getting your 
goat, if I keep running the program. 

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Let's see what happens if we try that with our 
set. I'll add the code to do that, on line 13: 

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I'm not going to type the print statement, 
because I've already got a warning from my IDE. 

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I'll hover over the warning, so we can see it: 

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The warning is a bit cryptic. Don't worry – 
the way it's phrased will make perfect sense, 

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after you've watched the OOP section.
I'll run the program. We'll get an error, 

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but the error message is more understandable.
TypeError: 'set' object is not subscriptable 

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not subscriptable is another way 
of saying that you can't index it. 

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You can't use an index with a set.
Which makes sense, now that you know that 

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sets are unordered. If you could ask for the 
item at position 3, which item would you get? 

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It might take many attempts 
before you eventually got "goat". 

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A program that you have to run many times, in the 
hope that you eventually get the right answer, 

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isn't much use to anyone.
So Python doesn't let us index into a set. 

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That also means we can't slice a set.
The first implication of sets being unordered, 

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is that you can't index or slice a set.
The second implication also won't be 

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surprising, once you've seen it.
I'll comment out those two lines, 

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to remove the error, and type 
the code to create a new set:

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Ok, more_animals contains the 
same items as farm_animals. 

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They're just in a different order.
If ordering isn't important with sets, then these 

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two sets should compare equal. Let's see if that's 
true. I'll add the code to test if they're equal: 

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Run the program, to prove that Python 
considers the two sets to be equal. 

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The sets are equal.
Python considers sets to be equal, if they 

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contain the same items. Ordering is unimportant 
– it's the items contained in the set that count. 

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That's different behaviour to lists and tuples. 
Lists and tuples will compare equal if they 

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contain the same items, in the same order.
You now know a couple of ways that sets 

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differ from lists or tuples. What you may 
be wondering is, why bother using a set? 

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Sets seem to be similar to lists and tuples, 
but more restricted. So why should you use them? 

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In the introduction to this section, I said 
that a set can be used to fix a bug in one 

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of our early programs. We'll have a look 
at that next. See you in the next video.