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Man: Welcome back.

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So we've just learned about

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the first two, well kind of three because we also mentioned

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that there is such a thing as a complex number in Python.

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But we talked about the main ones, the int and float.

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And there are still a few data types remaining.

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But before we get to these,

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we are missing an important concept.

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And this is going to be our first important term in Python.

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And, as a matter of fact, in all programming languages.

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It's called, variables.

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Yep, that's a term.

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Now, if this is your first time

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learning a programming language,

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you might not know what this means.

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If this isn't your first time, well, this is very simple

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because all languages have variables.

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But what are they exactly?

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Well variables store information

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that can be used in our programs.

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So we can hold perhaps user inputs like values.

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Maybe when you log into Facebook, you need to

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hold some information such as your profile picture.

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Or maybe your date of birth in a variable.

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Variables are ways for us to store information

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on our computer.

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So let's have a look at this.

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If we remove this,

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and let's say I'm creating a quiz program,

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and this quiz program maybe measures your IQ,

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and let's say you just took the quiz,

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and you found out that your IQ is 190.

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Quite high, good job.

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But we need to store that information somewhere.

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Well, we can do that with variables.

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So that in Python all we need to do is

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name it whatever we want.

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In our case, it'll be "IQ."

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And we're gonna say, "IQ equals 190."

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And this IQ here is a variable.

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It is something that I just completely made up.

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I could name it whatever I want.

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That's a variable too.

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The idea is that once we assign to a variable, that is,

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we're saying 190 is going to be assigned to IQ.

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I can now use it in my program whenever I want.

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For example, I can later on print,

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oh, make sure it's not cap.

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Let's do print, IQ.

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And if I do that and click run,

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you see that I can use IQ.

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So we can pretend here that a user takes a quiz,

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finds out their IQ is 190.

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We can store that information in this variable

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and later on when they come,

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perhaps online or try to use the program again,

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they don't have to take the quiz all over

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because, well, we store that information in IQ.

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And remember what I said at the beginning,

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programs are simply data that's being stored,

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that's being changed, that's being removed,

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and that's all programs are.

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And variables are important concepts in Python

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and all languages.

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Now, variables can also sometimes be called names.

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So, this could be a name for example.

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And assigning a value,

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is also known as binding.

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That is we're binding the value 190 to this variable,

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so that when we request this variable

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later on in our program,

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our computer knows how to look for this information.

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It's going to say, "Hey, I know what IQ is.

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I stored it somewhere in memory."

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And it's going to go look for that.

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And because it's being bound to a value

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it points to this value 190.

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And remember, this number in memory gets stored

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as a binary representation in zeros and ones, right?

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But it doesn't matter to us because

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however our machine stores it, we don't care.

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We just wanna be able to retrieve it.

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And then when we print it to do, well get 190.

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Now, we're gonna be using variables all over the course.

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But on top of just naming variables however we want,

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there is some best practices around variables

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of how you should write good variables.

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And as a matter of fact,

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these are specific rules that the Python community

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as a whole has, that you'll just have to remember.

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So let's have a look at this.

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Variables, and remember, this is the symbol

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for best practices, are what we call snake_case.

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Snake_case means it's all lower case, and then spaces,

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while they don't exist, we use underscores.

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Variables must start with a lowercase or an underscore.

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Variables can be anything with letters,

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numbers, and underscores.

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But remember, they have to start

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with lowercase and underscore.

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That means we can't start a variable with a number.

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They're also case sensitive.

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That means if I create a variable,

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but let's say this snake_case,

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this variable, has a capital E instead of a lowercase E,

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that'll be a different variable.

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And then finally, you can't overwrite keywords.

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Let's go through these with some examples.

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First, a variable has to be in the form of a snake_case.

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That is, if I wanna call this "user IQ,"

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I should technically have an underscore here.

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Instead of a space, just to make sure that a programmer,

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maybe I'm working on a team, can read this variable.

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So that's snake_case.

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You also have to start your variables with either a letter

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or an underscore.

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So I can technically do this.

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And I click run.

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Well, that's going to gimme an error

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because I've changed the variable.

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So now, in order to access that variable,

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you have to go like this.

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Now, underscore in Python signifies a private variable.

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Something that we'll go over later on in the course.

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But usually you're starting your variables with a letter.

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And afterwards, yeah, you can add numbers

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if you want in here, that's no problem.

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This is still going to work.

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This is a valid variable.

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Finally, a variable is case sensitive.

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So if I do "user IQ" here,

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and I do capital letters,

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I can't access this,

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like this because, well, it doesn't exist.

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Again, it's case sensitive.

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We have to make sure that we match.

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And finally, we don't wanna overwrite keywords.

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What does that mean?

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Keywords in Python?

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Well, they already mean something in Python.

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For example, this print is a keyword.

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You can see it highlighted in blue.

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So that if I create a variable

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saying print equals 190,

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and then I do print, print?

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Hmm, let's see what happens.

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We get an error because I can't really assign

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to this variable because print already means something.

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Now I know what you're wondering.

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What are these keywords in Python?

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That's a simple Google search away,

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and we'll learn these throughout our course.

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If we go to Python keywords by W3Schools,

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you'll see that we have these keywords

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that each means something in Python.

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Again, we'll go through these

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and we'll learn them throughout our course.

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And if you look, it's not that intimidating.

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There's not that many.

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So as we practice, you'll start to get familiar with them.

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But the easiest way to tell whether it's keyword

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or an important word in in Python,

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well you see that it's highlighted in blue.

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As soon as you create a variable that is unique,

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it's highlighted in white.

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And this will be the case with whatever

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environment that you're typing code into.

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As long as it's set up for Python.

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Now, beyond the Python keywords, there are different things

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like for example, the i.n.t,

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for integer that we've learned.

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So these we're gonna get familiar with.

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So, a good rule of thumb for variables

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is to make them really descriptive.

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Really say what your intention is.

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And a good programmer is somebody that's able to

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name things really well with their variables.

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So if a new developer comes and looks at your code,

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it's easily understood.

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Finally, variables can also be reassigned.

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For example, let's say we have IQ here of 190,

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and then I decide to, perhaps, have another variable,

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call it "user age."

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And for some reason I wanna assign

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user age to perhaps have

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IQ divided by four.

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Is that going to work?

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It should, right?

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I'm saying user age is going to equal 47.5.

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I'm using IQ, which is 190,

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dividing it by four,

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and then assigning it to user age.

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I can maybe assign this to another variable

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called "user age or called a",

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and once again, I run this

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and it's printing the same thing.

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So you can use variables to store that information

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and use it whenever you want.

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You can use it in operations, you can use it to reassign it,

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whatever your program needs.

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Now, later on in the course, we're gonna learn about classes

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and classes actually have a different convention than this

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but we'll get to that later on.

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For now though, I wanna mention two small gotchas

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with variables that you should be careful with.

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For example, there's an idea of constants.

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And constants are those things that

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never change in a program.

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For example, if we wanted to create a constant

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such as the value of pi,

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let's say for now it's 3.14,

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we can have it all in capitals.

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And that's going to tell other programmers that,

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"Hey, this is a constant."

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This number is not meant to change.

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I mean, we could change it if we want.

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There you go.

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I just made pi equal to zero.

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So it was stored as 3.14 in memory

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but then we overwrote it and reassigned it,

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the value of zero.

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You can still do that but a good convention

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is that if you see this, that means this number

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or this value, should never be changed.

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Another type of variable that you're gonna see,

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and this is something we'll see later on in the course,

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are the, it doesn't look like

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I just did double underscores here,

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but it's two underscores, and we call these dunder.

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And as you can see here, we have some

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dunder variables that Python has.

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Now, we'll learn more about these later on

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but, the idea here is that these

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are meant to be left alone.

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You should not touch them.

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You shouldn't create a variable with two underscores,

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like this, and call it "hihi" and assign it a value.

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I mean, you still can, however,

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this is generally not good practice

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so you wanna be careful with that.

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But the one thing that I really want you to take away

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from this, is that variables

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are really important concepts in programming.

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Naming variables is one of the most important skills

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you have as a programmer.

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I know it sounds silly, but, there's so many times

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that I read code that is so hard to understand,

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simply because a programmer is not descriptive enough.

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So throughout the course, we're gonna learn how to

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name things well, so that our code reads like English.

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And that's the whole point of writing good code.

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The point of writing good code is that

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it's readable and understandable by other programmers.

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By the way, to finish off,

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I just wanna show you one quick trick.

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There's also a way that you might see, in some code bases

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that uses something like this.

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Equals 1, 2, 3.

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And this simply is a way for us to rapidly

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assign values to variables multiple times.

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So for example, if I do print A, then print B,

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then print C, and I run this,

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you see that I get 1, 2, 3.

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We assign value of one to a, value of two to B,

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and value of three to C.

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Just a quick shorthand way that you might encounter.

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All right, let's take a break and I'll see in the next one.