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Our loop, from the last video, isn't 
very efficient. That's because it 

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creates a new set, each time round the loop.
If we check the documentation again, the union, 

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intersection, difference, and symmetric_difference 
operations all return a new set. 

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This part of the documentation lists the methods 
(and operators) that can be used with both sets, 

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and frozen sets. Frozen sets are immutable sets.
Because they're immutable, there are no methods 

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here that update a set.
Scrolling down a short way, 

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there's a list of operations that apply only 
to sets – you can't use these with frozen sets. 

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The first one is update.
For consistency with the three 

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methods that follow it, it should probably have 
been called union_update. Because that's what it is.

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It performs a set union, but modifies the 
original set, rather than creating a new one. 

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If we change our code to use the update 
method, or the update operator, we can avoid 

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creating a new set each time round the loop.
I'll comment out line 19, and use update: 

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Run the program, 

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to see the same result as we had last time.
That's more efficient. We update an existing set, 

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instead of creating a new one each time.
Sometimes, you will want to create a 

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new set. In that case, you'd use the 
union method – or the union operator. 

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Here, we were effectively 
updating the meds_to_watch set, 

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so it makes more sense to use the update method.
As with all these set operations, 

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there's also an operator to do the same thing. 
I'll comment out line 20 and use the operator: 

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When we covered augmented assignment, back in 
the Program flow control in Python section, 

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I mentioned that there were a few 
more augmented assignment operators. 

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This is one of four more, 
that we can use with sets. 

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Ok, I'll run that to check, before we move on.
and we get the same result. 

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In the next video, I'll discuss 
the advantage that the set 

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operation methods have, over the operators.
Before you start watching the next video, 

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create some sets of your own, and practise 
with the union and update operations. 

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For example, you might create these sets:
and create a set of things that bite, 

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or things that sting. Spiders 
bite, scorpions and vespas sting. 

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Spiders and scorpions are arachnids, 
so you could get a set of arachnids 

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by forming the union of those two sets.
Experiment, and I'll see you in the next video.