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Different Update Methods

00:00 Let’s look at some more set modifying operations. All of the ones in this video will have a method and a corresponding augmented assignment. They will also correspond to some of the set operations—union, intersection, difference, and symmetric difference.

00:19 Starting with update, this modifies the set by adding any elements that do not already exist. It’s very similar to union, where that will create a set with all elements in all sets. So instead of creating a new set, we actually change the set to include all elements.

00:37 Here’s the method, .update(). It takes in multiple arguments, and they need to be iterables.

00:43 Here is the augmented assignment. The operands need to be sets. This is different than the operator that you saw in previous videos because the operator creates a new set while augmented assignment modifies the existing set x1. Let’s look at some examples.

01:02 x1 is a set with {'foo', 'bar', 'baz'}, x2 is a set with {'foo', 'baz', 'qux'}. We use the augmented assignment to change x1 to include all the elements of x2. So then x1 will be {'qux', 'foo', 'bar', 'baz'}.

01:20 Notice how 'foo' doesn’t appear twice, as well as 'baz' doesn’t appear twice. And it is unordered, so they just appear in some random order.

01:31 Here we can also update x1 using the method and pass in the list of ['corge', 'garply'], and that will update x1 to now include those two strings as well as all the other elements it originally had.

01:47 Intersection update corresponds with intersection and modifies a set by retaining only elements found in both sets. .intersection_update() takes in multiple arguments, and here is the augmented assignment, ampersand equal (&=).

02:03 So here’s our example. We have {'foo', 'bar', 'baz'}, we have {'foo', 'baz', 'qux'}. We use x1 &= x2, and that will change x1 to include only the elements found in both sets, which are 'foo' and 'baz'.

02:19 We can call the method and pass in a list of ['baz', 'qux'], and we will only retain those elements that are both in x1 and in this list, which is just {'baz'}.

02:33 Moving on to difference update, which corresponds to the difference operation. It modifies a set by keeping only elements that exist in the first set, but not any of the rest.

02:43 Here we have x1.difference_update(x2[, x3...]), and this will modify x1 to keep only elements that exist in x1, not x2 or x3, et cetera. The arguments need to be iterables.

02:56 Here is the augmented assignment, -= x2, and then we actually have union| x3 | x4. Operands need to be sets. Note, there is the union operator between multiple operands.

03:12 This will mutate x1 by checking the union of all sets, x2, x3, and remove any elements from x1 that exist in the union.

03:20 x1 -= x2 - x3 et cetera won’t do this, and we’ll see that in the example.

03:28 Here we have a is the set {1, 2, 3}, b is the set {2}, c is the set {3}. And then we call a.difference_update(b, c), we remove all the elements found in b or c from a, and that will give us the set {1}. Here we reset a. Remember, we’re using a mutation now, and so we actually have to reset a if we want to use the set {1, 2, 3}.

03:53 Then we do a - b | c. It will first evaluate the union b | c, which will be the set {2, 3}, and then remove those elements from a. We reset a, and let’s try to use the -=b - c.

04:11 That will evaluate b - c, which will find the difference of b and c, which will be the set {2} because, remember, difference finds all the elements in the first set that do not exist in the second set.

04:23 So that’s essentially doing a -= {2}, which will remove 2 from our original set. So this was not the same as calling .difference_update(b, c).

04:37 Symmetric difference update will modify a set by keeping only the elements that exist in a single set, but not in multiple. x1.symmetric_difference_update(x2) will modify x1 so that x1 only includes elements that exist in either x1 or x2.

04:54 The argument needs to be editable, and for some reason, it only takes in one argument. Here’s the augmented assignment, ^=. Operands need to be sets, and this will mutate x1 to include all elements found in either any of the sets, but not in multiple sets.

05:12 Here’s an example. We have a is {1, 2, 3}, b is a set {3, 4, 5}, c is a set {1, 5, 6}.

05:21 We call a.symmetric_difference_update(b). That will update a to include only elements found in either set, but not both. That will be {1, 2, 4, 5}.

05:34 Here we reset a, and then we use the augmented assignment ^= to update a to include only elements found in one set, but not multiple.

05:45 So here, those elements would be {2, 4, 6}, because 2, 4, 6 are only found in one set, but not multiple.

Avatar image for Sciencificity

Sciencificity on April 27, 2020

Hi James,

Here again I believe that the symmetric_difference_update() only works on 2 sets for a reason (hence the ^= operator too, should be used only with 2 sets)

If we take the 3 set example used in the videos, and introduce a 3 in the last set we notice the weird behaviour for more than 2 sets.

a = {1,2,3}
b = {3,4,5}
c = {1,3,5,6}

a ^= b ^ c

{2, 3, 4, 6} # result 

# 3 is in every set so should not be here in the result 
# 3 gets eliminated in the a ^= b step (result = {1,2,4,5}), 
# then reintroduced in the last ^c step ...

Well, that’s my thinking anyway. Please let me know if I am thinking about it incorrectly?

Thanks for a really great course! It is just this concept which is confusing for me.

Avatar image for James Uejio

James Uejio RP Team on April 29, 2020

@Sciencificity Thank you for the comment and this is definitely tricky. So what actually happened is Python evaluated the right hand side of the expression first (b ^ c) and then modified a.

You can think of it like this:

a = {1,2,3}
b = {3,4,5}
c = {1,3,5,6}

# step 1
print(b ^ c)
{1, 4, 6}

# step 2
print(a ^ {1, 4, 6})
{2, 3, 4, 6}

# step 3
Modify a to be equal to {2, 3, 4, 6}
Avatar image for Victoria

Victoria on March 27, 2023

The explanation in the video is not correct. A counter-example, by adding another set. 
a = {1, 2, 3}
b = {3, 4, 5}
c = {1, 5, 6}
d = {1} # additional set

a ^= b ^ c ^ d
# {1, 2, 4, 6}

The ^ operations are chained and evaluated on the right first, before the final operation and assignment.

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