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Variables
9:51Let's explore how to store the objects that you create for later use.
So you know that Hello, World string
that we used in our hello.py file?
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To be a little bit more specific
we created the new string.
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We did that creation by surrounding
our text with quotation marks.
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This is what is known as a string literal.
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Now, we currently don't keep that
string around, we create it,
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we use it in our print statement,
and then we let it go.
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But we can actually store it and
use it later.
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In the real world, we create things and
store them for later use all the time.
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Here's an example I can
think of right now.
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Actually I can't stop thinking though
probably because it is about lunch time.
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Last night I made some delicious tacos, so
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good in fact that I wanted to
bring them into work today.
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So I grabbed a food storage container,
some Tupperware, and
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I put the remainder of
the delicious meat in it.
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When I put it in the refrigerator
here at the office,
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I decided that I had better label it,
mostly so people knew it was mine.
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Putting a label on it ensures that no
one is just going to throw my food away.
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Also I know it's mine.
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We have a lot of other labeled
Tupperware in our fridge.
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Labeling it helps me find my food.
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Now, here's a little confession.
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I end up making a lot of tacos, and
I end up keeping a bunch of leftovers.
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And the problem that leads to is this,
if I just label this with my name,
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I don't actually remember
what is in the Tupperware.
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Now one way I get around that is
by adding another label to it.
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This one is beef.
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This one is chicken.
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Some carnitas and this one is something.
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So there are two of these labels
referring to the same object.
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In Python everything you create is
an object and you can label an object so
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that you can refer to it
later in your program.
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These labels that you create
are called variables.
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Variables allow you to refer to
objects that had been created.
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They are object references.
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Now let's launch our workspace and take
our example from Tupperware to software.
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Okay, it's time to make this
code a little more personal.
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Let's change this so it says hello to you.
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So I'm gonna come in here and
I'm gonna change this to be my name.
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I'm gonna type Craig.
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You should type your name there.
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And now, let's go ahead and
add another print statement and
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this time write it about yourself.
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So we'll do print, call the function,
and we'll say, Craig is learning Python.
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Now let's go ahead and
run the script, again, that's python and
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then you can type the start of your
file name and then you can press tab and
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then it will complete it for
you, nice right?
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Awesome, now, what if I asked you to
change who this program was addressing?
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Like instead of you,
some other clever person.
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Ooh, I know.
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Let's change this to greet the very
first computer programmer, Ada Lovelace.
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She published the first algorithm for
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a device called the analytical
engine in 1840s.
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Pretty clever as putting it lightly.
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Check the teacher's notes for more.
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So now, I know you could very easily just
go and replace your name with Ada's.
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But let's do something a little
more clever than that.
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How about we create a new string
with her first name in it?
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And then we store it.
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So we could use it later
in these lines right?
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These lines here.
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We can use it here and here.
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So, you know how to
create a string already.
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You just surround some text with quotes.
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So we'll put Ada's name in there, Ada.
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So, that creates a brand
new string object.
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That's a string literal.
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And we wanna keep this around so
we need to give it a label or a name.
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So let's see,
how would we like to refer to the string?
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What's a good name for it?
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Well, it is a first name,
so let's call it that.
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Now, there are some naming rules that
I've added to the teacher's notes.
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But the most important one is that there
can't be spaces in a variable name.
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So if we say first and
I wanted to call this.
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We'll move her over here.
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So we wanna say first,
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and then this is where I would say first
space name if I was just naming something.
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But we can't have spaces
in a variable name.
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So what you do,
is where you would normally put a space,
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you use an underscore.
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So we'll put underscore, that's next
to the zero key on your keyboard.
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I know that that's one that you
probably don't use very often.
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And then you put the next word so, name.
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And you assign object to
the label using the equal sign.
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This is called assignment.
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So the string Ada is assigned
to the first_name variable.
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And now the first_name variable refers to
this string object that we created, Ada.
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So let's use it.
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But one thing to notice is that we've
kept the variable name all lower case.
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Remember, case matters.
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And it makes a difference in
your variable names as well.
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We follow the typical
Python naming convention.
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You keep you variable names lower case.
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And you use underscores to separate words.
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A fun fact, this naming style
is often called snake case.
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More in the teacher's notes.
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So, first_name is our string, right?
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So we should just be able to use it.
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So lets see, if I come in here and
I just say print(first_name),
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let's go see, so I'm gonna use the up
arrow, let's see what that did.
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We should see Ada and then Hello, Craig.
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Awesome, it's right there.
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So, in order to fix these other
statements we're going to
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first explore a little something
about the print function.
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So the print function as we've used it,
is just taking a single argument, right?
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We're just giving it one argument here.
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Well, it actually takes
multiple arguments.
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Remember when I talked about the ellipses
just briefly when we looked at that
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help documentation?
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So, what happens is that each value that
you add to the print function is printed,
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one after the other,
separated by spaces by default.
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Like this, here.
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Here, check this out.
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So, if we come in here and we say print.
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Now, I'm just gonna create
a string called These.
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And then I'm going to add a new argument
and I do that by typing a comma.
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And now I'll type a new string,
let's say, will be.
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And I'm gonna close that string and
then I'll do one more so
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we can see that there's just
another argument with a comma.
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And joined together by spaces.
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So if I'm gonna save that and
if I just run that one more time,
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we'll see each of these strings one
after the other separated by spaces.
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So let's go ahead and
rewrite this line first, so
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that we can see them next to each other.
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Right, so I'm gonna go ahead,
give ourselves some space here.
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I'll say print, and
I want the first part to be the same.
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I want that Hello, that's kind of
gonna be the same no matter what.
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We'll say, Hello comma.
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But now I want to end my string.
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And then we want to show our first_name.
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So we're going to do a comma,
See there's a comma in the string but
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there's also a comma on the outside here,
cuz that's separating our arguments.
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So we're gonna add our next argument.
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We'll do first_name, and
then we'll close the function call.
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So we should see, Hello,
Craig, Hello, Ada.
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And we do, awesome.
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So let's get rid of some
of these other lines.
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So I'm gonna come here and
in the Edit menu if you come here.
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You can see that there is a Delete Line.
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Which is Shift+Cmd D on a Mac.
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I believe it's probably
Shift+Ctrl D on Windows.
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So I'm gonna go ahead and press that.
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You'll see the line went away,
but if I come here and
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I do Shift+Cmd+D, it goes away.
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Now if you mess that up, there is always
Edit, Undo, and it will come back, right.
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And that was Edit, Undo is Cmd+Z.
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And there's a redo, Shift+Cmd+Z.
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So if I press Cmd+Z, it will go away and
Shift+Cmd+Z they'll go away again.
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Cool, so let's rewrite this
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learning Python using our
variable just like this is.
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So we'll say, print,
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and actually, why don't you go ahead,
and give that a go?
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Why don't you make that line,
use our variable so
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that it says Ada is learning Python.
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Pause me, and try to finish the line.
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When you're done, unpause me,
and I'll show you how I did it.
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Now don't worry,
you won't break anything, remember?
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Ready, Pause me.
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So how'd you do?
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Here's what i did.
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I used our variable first, right?
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So, I said, first_name, and
then I wanted to add another argument so
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I typed a comma and
then the remainder of the string.
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Now, I'm lazy so I just copied this, came
like this, and I did copy and I pasted it.
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Close that string here at the end and
close the call.
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There we go.
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And if we come down here,
we can clear this.
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We press up a couple times, there we go.
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Ada is learning Python,
I bet she'd love Python.
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One final thing that I'd like
to point out about our variable,
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is that you can actually label any object.
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So let's do this.
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Let's assign the variable
first_name a number.
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So let's say, 11.
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Right, so now we go and we run it.
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And it says 11 is learning Python.
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Not a common name, but
I've seen stranger things.
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11 would love Python too I'm pretty sure.
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Aren't you glad that we stored
that object for later use?
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It's just like those leftovers, mm.
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Ooh, that reminds me.
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I'm gonna go retrieve my tacos from
the fridge by using the label.
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After this delicious break,
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we'll talk about how to add a little
interactivity to our script.
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Taco to you soon.
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