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We've seen some of the basic theory of sets. 
Now let's see how that translates to Python code. 

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I'll continue with the DictAndSet project 
that we've been using throughout this section, 

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and I've already created a new 
Python file called set_intro.py. 

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Pause the video, while you 
create your new Python file. 

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There are several ways to create a set in Python – 
just like there are several ways to create lists, 

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dictionaries and tuples. I'll 
start by typing a set literal: 

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 A Python set is enclosed in curly 
braces – just like a dictionary. 

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Python can tell this is a set, rather than a 
dictionary, because we don't have key/value 

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pairs. For a dictionary, each item would 
be a key and a value, separated by a colon. 

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Here, we've got the 5 items that 
make up our farm_animals set. 

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We've used curly braces, on line 1.
If we'd used square brackets, then 

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we'd have a list. And if we'd used parentheses 
– or no brackets at all – we'd have a tuple. 

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Pay careful attention to the kind of brackets, 
when you're typing in your code. If you use 

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the wrong kind of bracket, you won't be 
creating the type of object that you expect. 

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I use a large font when recording these videos, 
but it can still be hard to see the difference 

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between parentheses and curly braces – especially 
if you're watching these videos on a mobile phone. 

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Ok, let's run the program to see what we get. 

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Our set prints out with curly 
braces, to indicate that it's a set. 

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We've got 5 items in the set, which is what 
you'd expect. What you might not have expected, 

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is that the items aren't necessarily in the 
same order that I typed them – on line 1. 

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Run the program a few more times, and make 
a note of the order that the animals appear. 

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They probably won't appear in the same 
order as you can see on my screen. 

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The order changes each time you run the program.
I'm labouring the point, because it's important 

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to understand that sets are unordered.
I had to type them in some order, on line 1, 

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but that doesn't truly represent 
how the items are stored in the set. 

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It would be more accurate to represent 
the set of farm_animals like this. 

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That's just a collection – or group – of things. 

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That's more obviously unordered.
If it was ordered, what order would you use? 

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Would you go from left to right, and 
get cow, sheep, goat, hen, horse? 

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Or would you go from top to bottom, 
and get sheep, hen, cow, horse, goat? 

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Ok, back to our code.
Looking at line 1, the items 

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inside the curly braces are the items in our set.
When we create a set using a set literal – by 

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typing in the items in other words – we 
obviously have to type them in some order. 

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We can't put images into our Python code .
But the set isn't ordered. 

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Another way to demonstrate that, is to see 
what happens when we iterate over our set. 

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Sets aren't a sequence type – sequence 
types are ordered, so a set can't be 

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one – but we can still iterate over 
a set. I'll add the code to do that:

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Run the program a few times. 

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Iterating over the set is obviously working, 
but we don't get the same order each time. 

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Because you can't rely on the order, you 
probably won't iterate over sets very often. 

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Alright, you now know that sets are unordered. 
In fact, you're probably wishing I'd shut 

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up about sets being unordered .
I'll move on – with a question. 

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Have you thought about the 
implications of sets being unordered? 

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What might we not be able to do with a set?
And how does all this impact 

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on checking if two sets are equal?
I'll answer those questions in the next lecture. 

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I'll finish this video by pointing out 
something that you may have already worked out. 

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Because sets use hashes to store the items, 

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anything that you want to put 
into a set must be hashable. 

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That's exactly the same requirement 
that we had for dictionary keys. 

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If you try to put a list into a set, for example, 
you'll get an error – the same "unhashable 

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type" error that we talked about, in The 
dict documentation video, earlier in this section. 

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Although that's obvious, you 
may not have thought about it. 

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See you in the next video.