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Welcome back. There are two ways to add

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stuff to the end of a list one we've seen already,

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and illustrated here is to append.

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So, origlist on line one,

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gets assigned to a list of three things,

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and then on line three that list gets an additional item at the end,

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it gets "cat" added to the end.

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The second way, is using concatenation, the plus operator.

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So, on line one,

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we create the same list,

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but on line three we are now going to use concatenation, and then reassignment.

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So, we concatenate the original list with a new list,

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and the new list has only one item in it,

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and then we reassign that back to the same variable.

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So, origlist is going to end up being bound to a new list,

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that has four items 45,

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32, 88, and the word "cat".

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Now, in codelens, it's not actually going to show us that we have a new list here,

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because there's nothing pointing anymore to the old list,

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and so it doesn't bother to show it to us anymore.

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So, it'll look like we've just replaced the old list

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but it's actually a new list with four items.

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So, sure enough that's what we get.

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Now let's look at a little more complicated one.

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Here on line one again,

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we're creating origlist with the three values in it,

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and then we're just printing out some things to remind us of what those are,

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but then on line four we're doing something different.

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Instead of reassigning the concatenated list of origlist plus "cat",

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back to origlist, we're reassigning it to a different variable called newlist.

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So, let's see how this is going to play out in codelens.

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On line one, we assigned the list to origlist,

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and we're just printing out some stuff.

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Line four, we're making a new variable,

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and it's pointing to the newlist.

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So, remember before I didn't show us that the old list was still unchanged,

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here we're seeing that the old list with

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just three items in it is still bound to the variable origlist.

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Even though we did a concatenation operator,

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that did not change the contents of the actual list.

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It made a newlist,

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and then we bound that to a new variable.

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So, then we print some stuff out,

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and now we actually are using an append operation.

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That's going to stick a new value,

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and additional value onto origlist, and so,

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we're going to get something added on here to origlist,

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you're going to get "cat" here as well.

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That's what we'll get. Let's run it and see.

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Sure enough "cat" is added on to that original list as well and then,

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we print some more things.

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Now we've printed out the identifiers,

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like these long numbers Just so you can see that

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origlist is pointing to one object with one identifier,

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and newlist is pointing to a different object with a different identifier.

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There's one more tricky thing to know about append and concatenate,

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and that is that, plus equals sign that we've seen previously.

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We've seen it as we had things like X,

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plus equals 1, and we described that as being a synonym,

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just another way of saying X equals X plus 1.

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In the world of mutable lists,

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that's not quite true anymore.

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They're not quite the same.

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Now, what's going to happen with the plus equal sign when we do it with a list,

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is that actually is going to mutate the list,

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it doesn't make a newlist,

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and concatenate to that.

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The only time that's going to matter is if we have some other alias for the same list.

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So, in this code, let's run it.

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The first line of code just assigns a list of three items to origlist.

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The second line of code makes a new variable that points to that same object,

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so aliaslist is an alias for origlist.

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Now we're on to line three.

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On line three we have this origlist, plus equals "cat".

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We're going see that that's a slightly different thing,

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than saying origlist equals origlist, plus the new thing.

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The difference is that we're actually going to extend the original object,

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and because we extend the original object rather than making a copy and a new one,

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we will also affect any aliases for it.

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So, when we run this,

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we will find that not only does origlist now have "cat" as its last item,

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but so does aliaslist.

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So, this is a different outcome than we would have

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gotten had we run this line of code instead of line three.

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My suggestion is, with lists don't use the plus equal operator it's just too confusing.

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Stick with what we have on line four,

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where we say origlist equals origlist, plus another list.

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Don't use the plus equal operator with lists.

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On the textbook page about append versus concatenate as with many of the other pages,

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there are lots of useful exercises,

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and I encourage you to try those out,

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and we'll see you next time.