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Welcome back.

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So we've just learned about the first two.

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Well, kind of three, because we also mentioned that there is such a thing as a complex number in Python,

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but we talked about the main ones that int and float and there are still a few data types remaining.

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But before we get to these, we are missing an important concept.

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And this is going to be our first important term in Python and as a matter of fact, in all programming

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languages.

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It's called variables.

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Yep.

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That's a term.

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Now, if this is your first time learning a programming language, you might not know what this means

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if this isn't your first time.

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Well, this is very simple, because all languages have variables.

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But what are they exactly?

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Well, variables store information that can be used in our programs so we can hold perhaps user inputs

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like values.

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Maybe when you log into Facebook, you need to hold some information such as your profile picture or

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maybe your date of birth in a variable.

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Variables are ways for us to store information on our computer.

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So let's have a look at this.

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If we remove this 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 and you found out that your IQ is 190 quite high.

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Good job, but we need to store that information somewhere.

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Well, we can do that with variables so that in Python, all we need to do is name it whatever you want.

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In our case, it will be IQ and we're going to 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, we're saying 190 is going to be assigned to

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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 or 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, you see that I can use IQ.

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So we can pretend here that a user takes a quiz, 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 perhaps online or try to use the program again, they don't have to take

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the quiz all over 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, that's being changed, that's being removed, and that's

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all programs are.

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And variables are important concepts in Python 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 is also known as binding.

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That is, we're binding the value 190 to this variable so that when we request this variable later on

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in our program, 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 and it's going to go look for that.

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And because it's being bound to a value, it points to this value.

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190 And remember, this number in memory gets stored as a binary representation in zeros and ones,

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right?

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But it doesn't matter to us because however our machine stores it, we don't care.

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We just want to be able to retrieve it.

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And then when we print it to do well, get 180.

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Now we're going to be using variables all over the course.

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But on top of just naming variables however we want, 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, these are specific rules that the Python community as a whole has that you'll

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just have to remember.

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So let's have a look at this variables and remember this is the symbol for best practices are what we

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call snake case.

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Snake gas means it's all lowercase and then spaces well they don't exist.

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We use underscores variables must start with a lowercase or an underscore.

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Variables can be anything with letters, numbers and underscores.

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But remember, they have to start with lowercase and underscore that means we can start a variable with

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a number.

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They're also case sensitive.

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That means if I create a variable, but let's say this snake case, this variable has a capital E instead

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of a lowercase E, they'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 want to call this user IQ, I should technically have an underscore here instead of a

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space just to make sure that a programmer 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 or an underscore so I can technically do

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this and I click run.

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Well, that's going to give me an error because I've changed a variable.

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So now in order to access that variable, you have to go like this.

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Now underscore in Python signifies a private variable, something that we'll go over later on in the

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course.

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

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Yeah, you can add numbers if you want in here.

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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 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 want to overwrite key words.

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What does that mean?

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Key words in Python.

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Well, they already mean something in Python, for example.

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This print is a keyword.

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You can see it highlighted in blue, so that if I create a variable saying print equals 190 and then

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I do print.

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Print.

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Hmm.

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Let's see what happens.

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We get an error because I can't really assign to this variable because print already mean 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 and we'll learn these throughout our course.

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If we go to Python keywords by three schools, you'll see that we have these keywords that each mean

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something in Python.

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Again.

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We'll go through these 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 or an important word in in Python, well, you see

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that it's highlighted in blue.

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As soon as you create a variable that is unique, it's highlighted in white.

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And this will be the case with whatever environment that you're typing code into as long as it's set

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up for Python.

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Now, beyond the Python keywords, there are different things like for example, the int for integer

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that we've learned.

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So these we're going to get familiar with.

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So a good rule of thumb for variables is to make them really descriptive, really say what your intention

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is.

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And a good programmer is somebody that's able to name things really well with their variables.

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So if a new developer comes and looks at your code, 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 and then I decide to perhaps have another variable,

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call it user.

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Age.

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And for some reason I want to assign user H to perhaps have IQ divided by four.

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Is that going to work?

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It should.

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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, dividing it by four and then assigning it to user age.

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I can maybe assign this to another variable called user H or called A and once again.

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I.

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Run this and its printing the same thing.

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So you can use variables to store that information and use it whenever you want.

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You can use it in operations.

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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 going to learn about classes.

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And classes actually have a different convention than this, but we'll get to that later on.

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For now, though, I want to mention two small gotchas 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 never change in a program.

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For example, if we wanted to create a constant such as the value of PI, 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, 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, so it was stored as 3.1 for a memory.

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But then we overrode it and reassigned it.

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The value of zero.

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You can still do that.

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But a good convention is that if you see this, that means that this number or this value should never

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be changed.

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Another type of variable that you're going to see and this is something we'll see later on in the course

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are the.

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It doesn't look like I just did double underscore here, but it's two underscores and we call these

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dunder and as you can see here, we have some dunder variables that Python has.

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Now.

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We'll learn more about this later on.

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But the idea here is that these 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 high.

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High.

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And assign it a value.

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I mean, you still can.

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However, this is generally not good practice, so you want to be careful with that.

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But the one thing that I really want you to take away from this is that variables are really important

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concepts and programming.

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Naming variables is one of the most important skills you have as a programmer.

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I know it sounds silly, but there's so many times that I read code that is so hard to understand simply

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because a programmer is not descriptive enough.

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So throughout the course we're going to learn how to name things well so that our code reads like English.

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

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The point of writing a code is that it's readable and understandable by other programmers.

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By the way, to finish off, I just want to show you one quick trick.

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There's also a way that you might see in some code bases that uses something like this.

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Equals one, two, three.

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And this simply is a way for us to rapidly assign values to variables multiple times.

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So, for example, if I do print A, then print B, then print C and I run this.

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You see that?

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I get one, two, three.

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

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Let's take a break and I'll see you in the next one.