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The set difference is the set of all items 
that are in one set, but not in the other. 

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I'll start with an example, using our odds 
and primes sets from a couple of videos ago. 

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I've created a new Python file called 
set_diference.py. Pause the video while 

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you create your file, and copy the first 
12 lines from set_intersection.py into it: 

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Ok. If we want a set of odd numbers that aren't 
prime, we take the difference of the 2 sets. 

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I'll use the difference method first, then 
we'll do the same thing using the operator. 

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Line 14 will remove all members of 
primes from the odds set. Run the program 

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and we're left with odd numbers that aren't 
prime. That's the last line of the output. 

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Prime numbers, in case you don't 
know, are integers greater than 1, 

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that aren't the product of two 
smaller numbers. In school, 

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they're often described as numbers that 
are only divisible by themselves and 1. 

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They're very important in programming, in 
areas such as security and cryptography. 

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Using the operator gives the same result:

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Run the program again,

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and we get the same set.
Just like with numerical subtraction,

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set difference is non-commutative. If we 
perform the operation the other way round,

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we get a different result. Let's see that:

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Run the program again.

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This time, we've removed the odd 
numbers from the set of primes.

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odds was the set of odd numbers less 
than 50, so there are no odd numbers, 

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less than 50, in the result.
In fact, the only value less than 50 is 2. 

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2 is counted as a prime 
number – it's the only even prime number. 

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Alright. That's the set difference 
operation, which we've seen using 

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both the method, and the operator.
In the next video, we'll use an earlier example, 

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to see a practical application for set difference.
See you in the next video.