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Earlier in this section, when we looked 
at the discard method, we wanted to remove 

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restricted items from our packing list.
discard is useful when you want to remove 

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an item from a set, without worrying 
about whether it was present or not. 

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However, when you want to remove 
every item in one set, from another, 

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the set difference is more appropriate. 

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Open the packing_list.py file from that 
earlier video, and we'll make some changes. 

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We'd left this code using 
remove, which causes it to crash. 

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That's on line 53. I'll change that 
back to discard, so that the code works: 

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I'll run the program to make sure it works 
– and also remind ourselves what it's doing. 

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Enter 2, to travel by plane.
There's quite a lot of output, 

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so I'll move the Run pane to Right Top. 

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The restricted items have been removed from 
the packing list, which is what we wanted. 

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That's a good demonstration of the 
discard method, but it's not the 

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best way to perform this particular task.
When you want to remove everything that 

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exists in a set, you can avoid 
having to iterate over the set. 

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We do that by using the set difference operation.
I'll comment out that loop, and replace it with difference.

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 I'll use difference update, because 
we want to modify the original items set: 

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I've used the operator first, this 
time, just to mix things up a bit. 

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We'll do the same thing with the method, 
next. Before we do that, run the program: 

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Enter option 2, and once again, all the restricted 
items have been removed from our items set. 

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That's a lot more efficient. We've 
replaced a loop with a single line of code. 

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Using the method is very similar. 
I'll comment out that line, 

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and call the difference_update method instead:

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Run the program again,

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and enter 2 again. 

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We get the same result – all restricted 
items are removed from the items set. 

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If you find yourself looping over the items in a 
set, and performing some operation on another set, 

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check out the set operations.
They're very efficient, and can save 

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you writing loops. Writing less code can mean 
introducing fewer bugs, which is a good thing. 

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Also remember that you can pass any 
iterable to the operation methods. 

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In this case, the code would still work fine, 
if restricted_items is a list rather than a set. 

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The last set operation to look at is symmetric 
difference. See you in the next video.