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Let's take a look at a "dunder" method.

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Now, we've seen this already with a class, any class called

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'Test', and we said 'def __init__'

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This '__' is called a 'double underscore',

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or in Python terms, we call it a 'dunder'.

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And this '__init__' method here is called a 'dunder' method.

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And Python actually gives us a bunch of these to work with

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right out of the box.

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And in this lesson, we're going to cover three of those.

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So let's go ahead and create an example here where we have

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a book, 'class Book:', and 'def __init__(self,', and the book 'title', and

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total 'pages', I guess,

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and maybe the 'content' of the book

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I guess. This is just an example.

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So we do 'self.title = title',

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'self.pages =,

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let's cast this as an integer, 'int(pages)', and 'self.content'

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is going to be whatever the 'content' is.

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So let's create a new book in here.

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'harry_potter', a book you may or may not have heard of, this is

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going to be a new 'Book()', 'title = "Harry Potter and the Prisoner

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of Azkaban', or something. I don't know,

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is it 250 pages, something like that, and then you know some 'content' in here.

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And that could actually be the book contents if we wanted to. Okay.

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So now we have this new class instantiated.

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And if we just type 'harry_potter', we see that it's a 'Book'

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at some memory location.

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And if we do 'type(harry_potter)', it just says it's a 'Book'.

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So this one is okay.

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But what if you wanted a better name for this?

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What if you didn't just want it to be named whatever the

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memory location is? There is a nicer way to deal with it.

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And you actually do use 'class names' a lot.

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And so one way we do that, or really the way we do that

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is with a 'dunder' method called, string, '__str__',

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and so that's going to take 'self',

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and whatever we return in here is what this class is

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going to be called. So,

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"Test Book".

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Okay. So I'm just going to rerun these cells, and we're going to

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see that 'harry_potter' is still pointing to some memory

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location, and that's okay.

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But what we want to do is print this out.

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If we print this out, it says "Test Book".

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So let's go ahead and actually delete this, because I got

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one step ahead of us here.

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Rerun this. Again,

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when we print out 'harry_potter' without the 'dunder', '__str__'

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in there, it just points to a location in our memory.

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But if we put '__str__', because we have

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this in here, that is the string representation of this 'Book'.

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Now, what we can actually do is change this to be 'self.title'.

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Rerun that, that, and that,

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and this is called "Harry Potter".

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Now we can start another book.

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I'm going to get rid of 'type' here.

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And let's call it Lord of the Rings, 'lotr = Book(

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"Lord of the Rings". I don't know,

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it's a 900 page book? Sometimes feels like it.

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And it's got some 'content' in there.

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We can do 'print()', Lord of the Rings, 'lotr', and we now know what this class

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is, it's called "Lord of the Rings".

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We know it's a 'Book', because if we do 'type(lotr)',

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[no audio]

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it is a 'Book'.

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Now let's go ahead and add a little bit more to this.

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What if we wanted to get the total length of the book,

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like how many pages there?

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We could actually just use 'Book.pages',

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but what if we wanted to get the total length, the total number

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of words, for instance? So we could do 'def __len__(self):',

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and just as an example, let's return 150, just for funsies.

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We'll rerun that, that,

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and in here we can run 'len(harry_potter)', and it gives us 150.

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So now we can actually perform some logic in here.

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We can say 'words = self.content.split(' ')', and then

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return the total 'len(words)'.

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Now we actually need to have some words in here,

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and so I just filled this with 'Lorem ipsum',

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it's 25 words of 'Lorem ipsum'.

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No idea what it means.

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I hope it's not offensive.

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And when I rerun 'len(harry_potter), it's going to get the total

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content, which is this one here.

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It's going to split it by every single space, and then it's

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going to return the total number of words.

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And there we go.

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We have 25 words of 'Lorem Ipsum' in there. That's exactly what I

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put into it, 25 words of 'Lorem ipsum'.

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So these are just three 'dunder' methods that come with

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pretty much every class, and you can overwrite them to be whatever

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you want, and let's see what other methods there are.

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So let's add a new cell here.

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And in here, let's do 'harry_potter.', hit 'Tab',

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but then if you write an '_', you can actually see that

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there's 'class',

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'del', 'delattr', 'dict', 'dir', 'doc', 'eq', 'format', all sorts

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of stuff in here, and these are all magic methods.

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These are 'dunder' methods that come with Python.

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A few of these you're going to see a lot more than other ones though.

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And some of these you probably won't ever see.

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So 'class' you will probably see in a framework like Django,

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'del', or 'delattr',

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you'll probably see that in data science stuff where you need

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to free up memory pretty fast.

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'dict', is actually a pretty cool one.

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I use this one for debugging a lot, and this one just turns

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your whole class into a dictionary.

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So you've got 'title', 'pages', and 'content' in there.

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That's pretty cool. That's a good one.

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We also have 'dir', which will show you all sorts of stuff,

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all the attributes, as well as all of the methods that are available in here.

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So that's a good one to sort of discover what a class has

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if you have not written the class yourself. 'init', yep,

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we've used that one a lot. 'init_subclass' is a good one.

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'len' we've looked at already. 'repr',

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you might end up seeing that one as well.

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That's a good one to set, but

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that's a, that's a best practice that a lot of people, as far as

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I can tell, don't tend to follow but it's a good one.

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And 'str',

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so that's the string representation.

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That's the name of the current class.

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And, yeah, there you go. So 'dunder' methods,

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there are a lot of them, a lot of them you probably won't

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use on a day to day basis.

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Some of them you will.

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I use 'init' and 'str' almost every single day.

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I've used 'repr' quite a bit, and 'len' is a good one,

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but as a web developer, I use Django,

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and I don't really need to use that one too much, because it

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usually comes with classes.

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But should you ever need to do that, you can do that.

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So if you ever create a digital bookstore and you want to

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use the number of words as the total book length, you can do that, too.

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You can get the total number of words and the average number

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of words that a human reads per minute,

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and then you could say that a "Book's length is, in minutes,

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180 minutes", something like that.

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Anyways, there are uses for it, but these are three pretty

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popular ones that I thought I would like to just sort of

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introduce you to and get you a little more familiar with.