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Back in the Introduction to Python sets video, 

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we had two sets; a set of farm 
animals, and a set of wild animals. 

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We'll use these sets to see how we 
can create the union of two sets 

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The union of two (or more), sets is 
the set of elements from all the sets. 

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As the diagram shows, the union of farm_animals 
and wild_animals contains all the animals. 

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Let's see how to create a union, in Python. 

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I'll create a new Python file called 
set_union.py, and define our two sets: 

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Okay. If we want a set containing all the 
animals, we can create the union of the two sets. 

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Sets have a union method that does just 
that. Before we look at the code, 

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let's have a quick look at the documentation.
That's something I suggest you do, 

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before using any Python feature for the first 
time. I'll go to the documentation for sets. 

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The URL will be in the resources for this video.
The first part documents the methods that are 

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available for both set types — set and frozenset.
frozenset is an immutable version of set. 

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Other than being immutable, it behaves 
exactly the same as the set type. 

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I won't go through all these methods. 
You've seen the len function a few times, 

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and it should be obvious. And we've 
already covered set membership. 

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I'll be talking about subsets 
and supersets shortly. 

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At the moment, we're interested 
in the penultimate four methods, 

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starting with union. I'll scroll 
them up, so we can see them. 

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We'll look at intersection, difference and 
symmetric difference in the next few videos. 

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In this video, we'll see some uses for set unions. 

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Before I move on, notice that we can use either 
an operator – the pipe or vertical bar – or the 

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union method. The same for the other operations 
– you can use either a method or an operator. 

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We'll see examples using both, as 
we examine these set operations. 

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Alright, back to our code, and we've got two sets.
If we wanted to do something with all the animals, 

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we can combine the sets, to create the union:

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Run the program,

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and we get a set containing 9 animals. 
Those are the animals that appear in both sets –

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as I showed in the last slide.
When forming the union of sets, 

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it doesn't matter which way round 
you do it. Instead of calling 

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the union method of farm_animals, we can call 
it on wild_animals and get the same result: 

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Run the program again.
The animals might not appear in 

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the same order – and I don't have to mention why 
again – but they both contain the same animals. 

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The union of two (or more) sets is commutative. 

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That's a posh way of saying that it 
doesn't matter which way round you do it. 

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It's the same as addition and 
multiplication, in arithmetic. 

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a + b is the same as b + a.
2 * 4 is the same as 4 * 2. 

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We can also use the union operator, the pipe:

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Once again, when I run the program,

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we get the same animals in the set.
So which should you use, 

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the union method or the union operator?
Most of the time, 

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it doesn't matter. A mathematician might 
prefer the union operator, because that's 

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how they'd normally form set unions.
The methods have the advantage that 

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you don't have to remember what the various set 
operator symbols mean – the code is more readable. 

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There is one important difference between the 
methods and the operators though. I'll talk 

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more about that soon, when we've got some data 
that I can use, to demonstrate the difference. 

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We'll see an advantage that the operators 
have, and an advantage of using the methods. 

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Alright, that's how you can form 
the union of sets, in Python. 

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In the next video, we'll see a more 
practical application of a set union.