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Deletion
5:24Now that we can add stuff, we'll definitely want to make sure we can get stuff out of the list. Let's explore `del` and `pop`.
All right, so I've updated our master
list, it's in the teacher's notes.
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I'm gonna go ahead and copy it, and
paste it, so it's all written now.
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Here we go,
we have the Learning Python book first.
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Let's remind ourselves of
what our app here is doing.
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So I'm going to go to python -i,
for interactive mode,
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and open up wishlist, right.
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It's pulling the first book out,
it's printing the first item out.
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But I've already read that book.
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So I kind of want this off my wish
list before someone buys me an extra
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copy of it.
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I wanna remove it, so you might have
not seen the del statement before.
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Well, basically del which is short for
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delete allows you to essentially
remove a label from an object.
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It deletes the name not the object.
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So let's go ahead and use my delicious
lunch as an analogy one more time.
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So, let's create a new
variable called Craig's lunch.
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And we'll set the value to
the Unicode value of a taco,
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more in the teacher's notes about Unicode.
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So, to do that,
we're gonna use a named Unicode value.
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So, we're gonna say backslash capital N,
and then a curly brace, and
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the name of the Unicode object is taco,
that's what we want there.
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So if you look at this,
you can see a beautiful taco, there it is.
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Now, my taco left overs
are labeled with craigs_lunch,
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but I have a bunch of other
leftovers in the office fridge here.
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So, I also wanna label the type of meat.
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Now, these tacos that I made last night,
they are carne asada.
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So I'm gonna add an additional label.
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So I'm going to say
carne_asada = craigs_lunch and
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we'll see that carne_asada now
also points to that same taco.
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Both of these labels are on that taco,
they are referring to the same object.
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I can remove one of the labels by using
the del keyword, so let's remove my lunch,
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so scary, del craigs_lunch.
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And now you'll see if I type craigs-lunch,
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we get name error because it
doesn't know what's going on.
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It's like, what is craigs_lunch?
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I don't know what it is, cuz I deleted,
I removed the label, now the label's gone.
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But don't fear, my tacos still exist
under my old label of carne_asada.
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That's still around and there is my taco.
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Now, if I remove this label,
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our office manager Molly will come through
and do a quick clean up of the fridge.
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Anything without a label
gets garbage collected and
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it's thrown out,
since no one that's claiming it.
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More in the teachers note about how
garbage collection works in Python.
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List indices work very similar to this,
that is,
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that you can think of them as labels.
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So, I can use the del keyword to
delete an item at a certain index.
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But it is really just deleting the label,
not the actual object.
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Now, for example,
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I definitely don't want this first
book on my personal wish list, right?
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This Learning Python book,
I don't want it on my wish list, but
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I do wanna recommend it still.
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So I'll add a new variable,
called, let's clear this up here.
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So, I'll create a new variable called
recommendation and I'm going to
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go ahead and add a label to
that object of recommendation.
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So now it's got two labels,
it's got book[0] and recommendation,
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and now I can run a del
statement on book[0].
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And if you take a look at books,
you'll see that Learning Python
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is no longer there, but
my variable still has access to it.
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There is the Learning Python
that I've pulled out.
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And sometimes, you don't want
to keep the value around, right?
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You want it to be garbage collected.
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That is when del is a perfect choice,
it's great.
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Often times though,
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you don't want to do a removal from
the list, you wanna keep it in a variable.
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And when this is what you want,
pop is the list method for you.
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By default, if you just use pop,
it takes the last item from the list.
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So if we say books, and
that method is called pop().
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We're gonna pop the last item off there,
so we should get Hello Web App back.
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Now, see how the last item was returned?
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The repl printed it out, right?
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So I could have stored that in a variable.
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This style of removing the last item
is often referred to as LIFO, or
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last in first out, right?
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The last item that's added to the list is
the first one that comes out when you pop.
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Now you can also pop at a specific index.
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So if I wanted to say books.pop(0)
it will pop the first book up,
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so automate the boring stuff and
I look at books it's not there.
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And there we go,
number one in my wish list.
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I think I'm gonna buy this right now,
my wish has been granted.
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Great job getting items added,
inserted, and
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remove from a list. How are things feeling?
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Remember, if anything at all has you
perplexed, you are part of a wonderful
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community of fellow students,
they would love to chat with you.
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You could even pop a question off
that unanswered question list, and
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answer one for your classmates.
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So, now that we can manipulate
these lists, I think it's time
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to start taking a look at some typical
solutions that they provide, sound good?
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Let's talk through some common use
cases of this powerful data type,
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right after this quick break
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