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We've made a class that implements its own
string, integer, and float conversion.

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We've also made that class
able to do addition.

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Now let's see if we can make our
own class that acts like a list.

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Are these skills you'll need to
survive in the Python world?

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You'd be surprised how often you can
cleanly solve a problem by making your own

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iterable object.

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Okay, let's get into workspaces.

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So first, I think I wanna see
about making my own list-like.

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Now, I've already made a class and
a module here, called items.

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And this has a couple of
different things in it.

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We're gonna use these, probably, but
we're not gonna be editing that.

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I've also started this inventory.py
with a class called Inventory,

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and this is the thing I
wanna make into a list-like.

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So I think I do wanna have an inventory
of stuff that belongs to a character.

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And it hold their weapons,
and their money, and

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their magic potions, and whatever.

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We have our class Inventory,
let's start by adding some stuff to this,

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so that we can put things
into an Inventory.

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So let's do like, def __init__(self), and

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we're gonna say that our Inventory
has slots, and that slots is a list.

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And now let's do def add,
and we're gonna add an item.

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And let's do self.slots.append(item).

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All right, so pretty simple, pretty basic.

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I want to be able to check
the length of someone's inventory.

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I want to be able to like,
you have 10 things in your inventory.

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Or maybe I want to be
like a lot of games and

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only allow you to store 10 things or
15 things.

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So to do that, if I'm in Python,
I would use the len function, right?

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And to use the len function on my object,

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my object has to have a method
that's named __len__.

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Have you noticed the pattern to this?

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It's kind of how everything works?

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So I'm gonna return len(self.slots),
and self.slots is a list.

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It already has the ability
to get its only length and

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we're just gonna send that out and
pretend that that's our length.

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We're gonna be sneaky.

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So let's try this.

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Come down here to our shell,
we'll go into our RPG directory,

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go into Python and
I'll say from inventory import Inventory.

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And I'll say from items import Item.

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All right, so let's make a coin and
I'll say coin = Item('coin').

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And I'll say this is a gold coin
because we provide a name and

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the description to each of the items.

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And then my inventory will be
a new inventory instance, and

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then I'll say inventory.add(coin) and
if I say len(inventory),

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just in my instance,
not inventory.slots, I get 1.

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All right, so that's cool.

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Now what about checking to see
if an item is in the inventory?

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I should be able to say,
hey is this thing,

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whatever the thing is, in the inventory?

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And Python should tell me yes or
no, or well, True or False.

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And yeah, I could just check
inventory.slots, but I shouldn't have to

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know that there's an attribute named slots
that actually holds everything, right?

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I shouldn't have to know the inner
workings of a class in order to use it.

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Luckily, this is all self contained in
a single magic method named contains.

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So we'll go ahead and
we'll define contains right down here.

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And we take self, and

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we take item, which is the thing
that we're wanting to look for.

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So we'll just return whether or
not item is in self.slots.

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Now, can it really be that simple?

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What do you think?

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Well, let's test this one more time.

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From inventory import Inventory,
from items import Item,

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and sword = Item('sword', 'sharp').

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That's a good description, right?

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And inventory = Inventory().

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And then we'll say, inventory.add
the sword, add the sword to my inventory.

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Sword in inventory, True.

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I guess it is that simple.

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So contains actually does something kind
of interesting if contains doesn't exist.

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Python tries two other magic methods,
it tries iter and it tries getitem.

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These two methods aren't meant for
checking membership but

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they can be used for it.

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If iter exists the object is
considered to be iterable, and

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Python can loop through it, so it will.

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It will loop through it, trying to find
something that is equivalent to the thing

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that you have asked for.

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If iter doesn't exist, Python tries to use
getitem, which is the method that lets you

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pluck things out of a collection
with an index or a key, right?

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When we've done like list bracket zero.

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It checks a bunch of positive
indexes in order and

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tries to find one whose value matches
the thing that you're checking for.

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I think it'd be handy to be able
to loop through an inventory,

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now that we're talking about that.

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So to do that, we'll need to create
the iter method that I mentioned before.

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But I need to warn you,
we're gonna have to use a new keyword.

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And its full explanation doesn't
really fit into this course.

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I have,
of course included the documentation for

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it in the teacher's notes.

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So check those out.

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So, we're gonna say, for
item in self.slots yield item.

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What is this yield keyword?

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It's very similar in
how it works to return,

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which you should be used to by now.

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But it doesn't immediately end the
execution of the method, like return does.

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yield lets us send values
back out of the function, or

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the method, as they're available,
and keep on working.

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This construct, a function or

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a method that returns values as it works
them out, is known as a generator.

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We can actually simplify this by using
two keywords, what we have written here.

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We can say yield from item, or

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not from item, from self.slots rather.

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And that will do the same as the for
loop we just had.

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Since self.slots is a list,
Python knows how to iterate through it.

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Then, the yield from keyword combo
sends out one item at a time

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while Python is iterating
through self.slots.

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So, thanks for
saving us some work, Python.

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Let's test this out one last time.

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Pop down here and
I'll type python correctly.

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And I'll say from
inventory import Inventory.

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And I'll say from items import Weapon.

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Let's try this with something
slightly different.

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This time we'll make our sword a weapon
and we have to provide that it's a sword,

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that it's sharp, and
this is how much damage it does, so 50.

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And then we'll say inventory =
Inventory(), inventory.add(sword).

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And then we'll say for item in inventory:,

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print(item.description).

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And we should get out sharp, nice.

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A great thing about using a generator here
is that even if our inventory is huge, and

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I mean hundreds, thousands of items,
iterating through it will

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be memory efficient because we'll only
be dealing with a single item at a time.

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Wow, our class is doing
some pretty cool stuff.

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When you make custom classes that can
be used in the same fashion as built-in

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classes you make it
easier to use your code.

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You don't have to think of any special
cases like right, when I use a whiz bang I

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have to call the configure method and
then get all method and then.

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No, just add things to it,
sort it, get the length, etc.

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just like you would any other object.

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Okay, take a brain and body break
then come back to learn about taking

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control with Python's built in classes.