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[MUSIC PLAYING]

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CARTER ZENKE: Well, hello one and all,
and welcome to our short on instance

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variables.

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In a prior short on defining
classes, we saw a way

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to create a class or a template for a
package, one what we called, in fact,

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package.

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And down below, we
created several instances

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of that class of that template to
present individual packages here.

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So notice on lines 11 and
12, I have here some code

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that defines for me two instances of
this class that we called package.

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It's a bit like me creating a template
for something called a package

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and down below creating
two particular packages.

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Now, I did so as follows.

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I said down below that this
first package has a number one.

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The sender was Alice.

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The recipient was Bob.

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And the weight here was 10.

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But what actually happens as
we call some code on line 11?

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Well, we said we'd come back to
these lines, 3 through 6 here.

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So let's take a look at
exactly what's happening.

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Well, when I run some line
of code, like on line 11,

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I again am creating some new instance of
this class I've defined called package.

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And along the way, thanks
to Python, this method

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called dunder init is called.

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Now, we said here that it
takes as input self, which

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refers to the new instance of
the class that we are creating.

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And it seems like what I'm doing
here is taking in various inputs,

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like number, sender, recipient,
and weight, and assigning them

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to various attributes or
properties of this class,

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of this instance of the class here.

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So it turns out that
each of these things,

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self.number, self.sender, and so
on, are called instance variables.

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They're variables that belong
to some instance of this class.

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And they are defined for me
whenever I call package down below.

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So let's look at number, for instance.

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Number equals 1.

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Well, I'm now passing in to dunder init
this argument, number, with the value 1.

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And what happens to that number 1?

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Well, it actually gets assigned
as an instance variable

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within this instance of the class.

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So this instance here has a variable
called number and so on for sender,

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recipient, and weight.

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But let's see this in action here.

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I'll come down below, and let me try
to actually access these instance

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variables that I claim I
set up on lines 3 through 6.

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Well, I could imagine, let's say,
looping through every package

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that I have inside of this
list of packages, maybe

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printing them out along the way.

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Well, to do so, I could use a for loop.

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I want to print for each package I have.

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So I'll say for package
in the list of packages.

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And then I'll fill
this in a little later.

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Notice first that package will first
refer to the package on line 11

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and then, on the next iteration,
the package on line 12.

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So very simple here, let's
start off by printing something.

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We could print the package
as a whole, but I don't

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think we'd get a very good output.

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What I could do instead is try
printing, in this case, something

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like the instance variable that we
called number, just to start with,

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package.number.

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So if I want to access an instance
variable, I can, in this case,

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refer to the individual instance of the
class, package here, followed by dot,

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followed by the instance
variable's name,

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just like we set up above,
in this case, number.

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So if I try this--

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I'll try Python of packages.py.

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Hit Enter.

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We'll see 1 and 2.

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But just to hammer this home,
let me try instead 1 and 3

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and now hopefully see 1 and 3.

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So it does seem, as I
defined in dunder init,

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that the input to these
various parameters of package,

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these arguments here, they actually
get assigned as instance variables

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of each instance of my class.

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Now let's try accessing
more and maybe printing

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these packages in a prettier way.

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So here I can do this.

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I could go back to the format I had
before but now use a Python f string

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in the packages instance variables.

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I could say package and then
package.number, just like this.

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If I were to run this, I could see
package 1 and package 2, pretty good.

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What if I tried maybe the
same format from before?

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I could try colon, and
then I could enter in,

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let's say, the package's
sender to package.recipient.

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And let's see what we see here.

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Python packages.py.

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Package 1, Alice to Bob.

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Package 2, Bob to Charlie.

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And this is all happening by
accessing these instance variables

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that I set up in dunder init as well.

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Let's now add in the weight.

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I'll say comma package.weight.

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And we said the weight was,
in this case, in kilograms.

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I'll go ahead and now
try to run this again.

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I'll go ahead and go back to my
terminal-- python of packages.py--

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and we'll see a much prettier way of
formatting these individual packages.

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In fact, we can get the benefits
of the strings we saw, let's say,

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in the short on defining classes, which
allows them to be more prettily printed.

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But we also get the
benefits of this class here,

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this way of encapsulating
information now using

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these things called instance variables.

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This then was our short
on instance variables.

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And we'll see you next time.

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