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What if we want our classes to act like a list or a dict?
One nice thing about emulating built-ins is that you can have your cake and eat it, too. You can make a class that's iterable but not searchable, or vice versa. This gives you a lot of control over how your classes are used.
yield
Briefly, yield lets you send data back out of a function without ending the execution of the function. Here's an example:
def get_numbers():
numbers = [4, 8, 15, 16, 23, 42]
for number in numbers:
yield number
If we used this function, with something like numbers = get_numbers(), we'd have a generator object. This is a special kind of object that has a value, a pointer to the current index, and a __next__ method (ooh, special method!) that knows how to get the next item from the iterable. We can do next(numbers) and we'd get 4, then 8, then 15, and so on.
Since we're just returning values from an iterable, we can use yield from to skip the entire for loop:
def get_numbers():
numbers = [4, 8, 15, 16, 23, 42]
yield from numbers
If you want to read more about yield and yield from, here are some docs.
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We've made a class that implements its own
string, integer, and float conversion.
0:00
We've also made that class
able to do addition.
0:04
Now let's see if we can make our
own class that acts like a list.
0:06
Are these skills you'll need to
survive in the Python world?
0:09
You'd be surprised how often you can
cleanly solve a problem by making your own
0:12
iterable object.
0:15
Okay, let's get into workspaces.
0:17
So first, I think I wanna see
about making my own list-like.
0:19
Now, I've already made a class and
a module here, called items.
0:22
And this has a couple of
different things in it.
0:26
We're gonna use these, probably, but
we're not gonna be editing that.
0:28
I've also started this inventory.py
with a class called Inventory,
0:33
and this is the thing I
wanna make into a list-like.
0:36
So I think I do wanna have an inventory
of stuff that belongs to a character.
0:40
And it hold their weapons,
and their money, and
0:45
their magic potions, and whatever.
0:48
We have our class Inventory,
let's start by adding some stuff to this,
0:51
so that we can put things
into an Inventory.
0:54
So let's do like, def __init__(self), and
0:56
we're gonna say that our Inventory
has slots, and that slots is a list.
1:01
And now let's do def add,
and we're gonna add an item.
1:07
And let's do self.slots.append(item).
1:12
All right, so pretty simple, pretty basic.
1:17
I want to be able to check
the length of someone's inventory.
1:20
I want to be able to like,
you have 10 things in your inventory.
1:24
Or maybe I want to be
like a lot of games and
1:26
only allow you to store 10 things or
15 things.
1:28
So to do that, if I'm in Python,
I would use the len function, right?
1:32
And to use the len function on my object,
1:38
my object has to have a method
that's named __len__.
1:40
Have you noticed the pattern to this?
1:44
It's kind of how everything works?
1:48
So I'm gonna return len(self.slots),
and self.slots is a list.
1:50
It already has the ability
to get its only length and
1:57
we're just gonna send that out and
pretend that that's our length.
1:59
We're gonna be sneaky.
2:03
So let's try this.
2:04
Come down here to our shell,
we'll go into our RPG directory,
2:09
go into Python and
I'll say from inventory import Inventory.
2:14
And I'll say from items import Item.
2:19
All right, so let's make a coin and
I'll say coin = Item('coin').
2:24
And I'll say this is a gold coin
because we provide a name and
2:27
the description to each of the items.
2:31
And then my inventory will be
a new inventory instance, and
2:34
then I'll say inventory.add(coin) and
if I say len(inventory),
2:39
just in my instance,
not inventory.slots, I get 1.
2:45
All right, so that's cool.
2:49
Now what about checking to see
if an item is in the inventory?
2:52
I should be able to say,
hey is this thing,
2:57
whatever the thing is, in the inventory?
3:00
And Python should tell me yes or
no, or well, True or False.
3:02
And yeah, I could just check
inventory.slots, but I shouldn't have to
3:06
know that there's an attribute named slots
that actually holds everything, right?
3:10
I shouldn't have to know the inner
workings of a class in order to use it.
3:14
Luckily, this is all self contained in
a single magic method named contains.
3:17
So we'll go ahead and
we'll define contains right down here.
3:23
And we take self, and
3:28
we take item, which is the thing
that we're wanting to look for.
3:29
So we'll just return whether or
not item is in self.slots.
3:33
Now, can it really be that simple?
3:39
What do you think?
3:41
Well, let's test this one more time.
3:43
From inventory import Inventory,
from items import Item,
3:48
and sword = Item('sword', 'sharp').
3:54
That's a good description, right?
3:58
And inventory = Inventory().
4:02
And then we'll say, inventory.add
the sword, add the sword to my inventory.
4:06
Sword in inventory, True.
4:12
I guess it is that simple.
4:15
So contains actually does something kind
of interesting if contains doesn't exist.
4:17
Python tries two other magic methods,
it tries iter and it tries getitem.
4:23
These two methods aren't meant for
checking membership but
4:30
they can be used for it.
4:33
If iter exists the object is
considered to be iterable, and
4:35
Python can loop through it, so it will.
4:39
It will loop through it, trying to find
something that is equivalent to the thing
4:41
that you have asked for.
4:45
If iter doesn't exist, Python tries to use
getitem, which is the method that lets you
4:47
pluck things out of a collection
with an index or a key, right?
4:52
When we've done like list bracket zero.
4:55
It checks a bunch of positive
indexes in order and
4:58
tries to find one whose value matches
the thing that you're checking for.
5:01
I think it'd be handy to be able
to loop through an inventory,
5:06
now that we're talking about that.
5:08
So to do that, we'll need to create
the iter method that I mentioned before.
5:10
But I need to warn you,
we're gonna have to use a new keyword.
5:15
And its full explanation doesn't
really fit into this course.
5:19
I have,
of course included the documentation for
5:24
it in the teacher's notes.
5:26
So check those out.
5:28
So, we're gonna say, for
item in self.slots yield item.
5:29
What is this yield keyword?
5:36
It's very similar in
how it works to return,
5:38
which you should be used to by now.
5:42
But it doesn't immediately end the
execution of the method, like return does.
5:43
yield lets us send values
back out of the function, or
5:48
the method, as they're available,
and keep on working.
5:51
This construct, a function or
5:55
a method that returns values as it works
them out, is known as a generator.
5:57
We can actually simplify this by using
two keywords, what we have written here.
6:01
We can say yield from item, or
6:06
not from item, from self.slots rather.
6:12
And that will do the same as the for
loop we just had.
6:16
Since self.slots is a list,
Python knows how to iterate through it.
6:20
Then, the yield from keyword combo
sends out one item at a time
6:25
while Python is iterating
through self.slots.
6:30
So, thanks for
saving us some work, Python.
6:33
Let's test this out one last time.
6:36
Pop down here and
I'll type python correctly.
6:41
And I'll say from
inventory import Inventory.
6:46
And I'll say from items import Weapon.
6:50
Let's try this with something
slightly different.
6:52
This time we'll make our sword a weapon
and we have to provide that it's a sword,
6:55
that it's sharp, and
this is how much damage it does, so 50.
6:59
And then we'll say inventory =
Inventory(), inventory.add(sword).
7:05
And then we'll say for item in inventory:,
7:12
print(item.description).
7:17
And we should get out sharp, nice.
7:21
A great thing about using a generator here
is that even if our inventory is huge, and
7:24
I mean hundreds, thousands of items,
iterating through it will
7:28
be memory efficient because we'll only
be dealing with a single item at a time.
7:31
Wow, our class is doing
some pretty cool stuff.
7:36
When you make custom classes that can
be used in the same fashion as built-in
7:38
classes you make it
easier to use your code.
7:41
You don't have to think of any special
cases like right, when I use a whiz bang I
7:43
have to call the configure method and
then get all method and then.
7:47
No, just add things to it,
sort it, get the length, etc.
7:50
just like you would any other object.
7:53
Okay, take a brain and body break
then come back to learn about taking
7:55
control with Python's built in classes.
7:58
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