Writing and Running Python Code

0:00Hi, Sean here and welcome to this course where we're going to be learning all

0:04of the topics that

0:04you'll need to know to pass the PCEP exam that is Python certified entry level

0:11programmer.

0:12Now, there's going to be a lot of topics that'll be covered here, but

0:17essentially what we're going

0:17to be doing is learning all of the basics of Python in depth, right? So we're

0:23going to go into the

0:25syntax of Python, we're going to obviously be seeing how to do things like run

0:29Python in different

0:30contexts. And we'll also see how to build a few simple projects in Python among

0:36many, many other

0:37things that we're going to do in this course. So the first thing that we're

0:41going to take a look at

0:42here is the basics of writing and running a simple Python program. Right, the

0:50fact is that all of

0:51the stuff that we're going to learn here, right, all of the Python syntax, all

0:55of the details that

0:56we'll learn is going to depend on your ability to run Python code. So it's

1:01pretty important that you

1:03just right off the bat get used to running Python in different contexts. So

1:10first of all, if you

1:12don't already have Python installed, what you're going to want to do, I've just

1:16because there's so

1:17many different operating systems out there and the process for getting Python

1:21set up varies so

1:22much between the operating systems. I've included a link to a pretty good

1:26article that shows how to

1:28set up Python on pretty much any computer. So feel free to follow that link and

1:34walk through the

1:35steps in it. And once you've got Python set up, you should be able to follow

1:40along with me here. So

1:42feel free to pause the video while you do that. And well, assuming that you now

1:48have Python set up,

1:49the first thing, the first way that I'm going to show you to run Python is just

1:56in the terminal.

1:57And first of all, the the IDE that you see me using here is called visual

2:02studio code, or

2:04that's often abbreviated VS code. And this is different from just plain old

2:09visual studio. VS

2:11code is basically just a nice straightforward development environment that you

2:17can pretty much

2:18add any plugins you want for whatever programming language you're using. So I

2:22find that it's great

2:24for Python development. And really, it's just easy to use. So I highly

2:28recommend it if you have

2:30another thing that you want to use, whether that's for your job, right, if the

2:33company you're working

2:34at uses something else, feel free to use that instead. That's perfectly fine.

2:38But anyway, what

2:41you're going to want to do first in order to do this is open up a terminal. So

2:45my IDE has a built

2:47in terminal. And if you're using visual studio code, you can either open this

2:50up with control

2:52or command J, or you can just go to terminal and do new terminal here. All

2:58right, that'll open up

2:59that window for you. Cool. So inside this terminal, what you should be able to

3:04do is run Python three,

3:06just that command. And what that'll do is it'll actually open up a Python read

3:13evaluate print loop

3:15for you. All right, so this is there's an abbreviation for this in case you

3:19aren't familiar with this

3:20term, our EPL, basically just refers to a simple program where you can type in

3:28Python statements,

3:30right, or Python expressions, and hit enter. And you can see the results of

3:35those statements. So

3:36the simplest thing that we're going to do here is we're just going to print

3:41something out to the

3:42console. And in Python, the way that you do this is by saying print, and then

3:48inside the parentheses

3:50that you put after print, which is how you call functions in Python, something

3:54that we'll talk about

3:55in much more detail a little bit later, you're going to add a string. And in

4:00Python, you know,

4:03strings just represent some sort of text. In Python, you can use either single

4:07or double quotes.

4:08Again, we'll talk about this in more detail later as well. But we're just going

4:11to say something

4:12like hello, Python. And if you hit enter now, sure enough, you're going to see

4:18that what

4:19happens is the read evaluate print loop, right, the REPL, our EPL, is going to

4:27execute the code

4:28that we entered in. And it's going to print out the results, right? So the

4:32result of calling

4:34print hello Python is that this program simply prints out hello Python to the

4:39console.

4:40Now you can do pretty much anything you want in a read evaluate print loop, a

4:46REPL.

4:48So if we want to create variables, and once again, I'm going to talk about this

4:52in more detail later,

4:53I'm just kind of showing you more the mechanics of running Python code than

4:59actually getting into

5:00the syntax yet. But we can define variables. So we can do that like, we can do

5:04that by saying

5:05things like x equals five, and we can hit enter. And as you can see, this doesn

5:09't have any direct

5:11result, right, creating a variable doesn't have any output as did printing

5:15something out to the

5:16console. So we're going to say x equals five, y equals, I don't know, let's

5:21just pick another

5:22number six. And if we say x plus y now, well, that does have a result, and that

5:29is 11, right,

5:30it adds together the values of these two variables. So again, this is just the,

5:36this is just the

5:37basic read evaluate print loop, this is really great for doing things like

5:41running very simple

5:42Python programs or experimenting around with different Python statements, right

5:48, as you're learning

5:49Python syntax, this can be a great place to come and just kind of, you know,

5:54play around with

5:54different expressions, try changing things, see what happens, right? So you

5:58could say, you might try,

6:00you might see if spacing matters in Python, as far as like having space in

6:04between the variable

6:06name and the value. So you could come back here and try changing x by saying x

6:10equals seven with

6:11no spaces. And you could see that that works, right? So again, this is just a

6:16tool that you can use

6:18to run simple Python expressions and statements. So, you know, feel free again

6:25to as you're learning

6:26different pieces of syntax come, come back here and try them out. It's just, it

6:31's a little bit

6:31easier than having to write the Python code in a file and run it manually. And

6:37speaking of writing

6:38Python code in a file, that's the next thing that we're going to take a look at

6:42here. So now that

6:43we've seen how to use the Python REPL, which again, you can open up just by

6:48just by typing Python

6:49three. And by the way, you can exit this by saying exit with two parentheses

6:55after it, that's going

6:56to close this read evaluate print loop by calling this exit function. All right

7:03, well, if we want

7:05to write our Python code in a file, which very often we will just because this

7:09makes it a lot

7:10easier to reuse that code, come back to it later, make changes, etc. All you

7:16have to do is create a

7:18new directory. So I've created this directory called PCEP examples. And inside

7:24here, what we'll do is

7:25we'll just say something like my first program dot pi, all right, so that dot

7:33py extension is

7:36what Python files will always have. So if you see a file with dot pi, you'll

7:41know that it's a Python

7:42file. And inside here, what we're going to do is we're going to type, we're

7:47just going to write

7:47some simple Python code. So I'm going to take things up a little bit and define

7:52two variables. So we'll

7:54say x equals five, y equals six, just like I did in the in the read evaluate

7:59print loop. And then

8:03I'm going to actually print out the sum of those two by saying print x plus y.

8:11And in Python, or

8:14rather, when we're writing Python in files, as we are here, this print function

8:19is going to be

8:20especially important because unlike in the terminal, right, when we were using

8:25the

8:25REPL to run our Python programs, when we're running Python code in a file, it's

8:32not going to

8:33print out the results of every statement or every expression by default, right?

8:39It's not, you know,

8:40if we were to just say x plus y here, instead of print x plus y, oops, let's

8:45try that again,

8:45there we go. If we were to just say x plus y and run this code, it wouldn't do

8:49anything. So by

8:50adding that print function, that's actually going to print that out to the

8:55console where we can see

8:56it. These are just little details that you'll get used to as you work with

9:00Python more and more.

9:01So don't worry too much about remembering all of them right now. But anyway,

9:05now that we've written

9:06a little bit of Python code in this file, let's run it. And you can run Python

9:12files by saying

9:13Python three, right? So that same command as the way that we ran or the way

9:18that we opened up that

9:20REPL. And then after that, you're going to put the name of the file that you

9:25want to execute. So

9:26I'm going to say my first program dot pi and make sure you write all of that

9:30correctly. If you change

9:31a letter to it won't work. So if we had enter, sure enough, we see that that

9:35will print out 11,

9:37right? x plus y here to the console. And just to demonstrate what I said before

9:42, if we were to

9:42just say x plus y without the print function, and run it again, sure enough,

9:47you'll see that

9:48nothing actually gets printed out. So again, when you're running Python in a

9:51file, the only

9:54things that are going to get printed out that we write from our code are things

10:00that we specify

10:01with that print function. Okay, so let's just add that back and run our file

10:05again to make sure

10:06everything is looking good. So that's the second way to run Python code. And

10:11really, these two ways

10:13are going to be are going to be are going to form the majority of the examples

10:17that we'll do in this

10:19course. There are other ways, though, one way, which you've undoubtedly heard

10:24people talk about

10:25is by using something called a Jupiter notebook. All right, Jupiter with a

10:31clever little pie here

10:32right Python, or py for Python that is, this is sort of like, this is sort of

10:40like what we saw with the

10:42read evaluate print loop in the terminal. But it's much more interactive and

10:47visual. So we're

10:48actually going to be going through just a we'll do a skill about how to work

10:52with Jupiter, just

10:54the very basics of running Python in a Jupiter notebook a little bit later in

10:58the course. So if

10:59you're curious about that, feel free to jump ahead and take a look at that

11:03skill. But anyway,

11:07that's one more way that you can run Python code and a very common way at that,

11:12especially if you're

11:12getting into something like data science. But you can also use there's plenty

11:18of online interpreters.

11:20And by the way, this word interpreter, we'll talk about that in a little bit

11:25more detail later. But

11:27this is Python itself, unlike languages like C++, which are compiled languages,

11:35Python is

11:36actually what's called an interpreted language, right, which basically means

11:40that this code that

11:42we're running doesn't get converted into a bunch of binary before running it.

11:48As would be the case

11:50with C++, right? So with C++, you talk about the compiler with Python, you talk

11:55about the

11:56interpreter. So in other words, when we run Python three with the file name, we

12:01're telling the Python

12:02interpreter to execute the code that that file contains. All right, but anyway,

12:07there's lots of

12:08online interpreters, one that I like, just for basic demonstrations is called

12:14replit.com.

12:15And it's pretty nice, especially if you're having trouble getting Python set up

12:20on your

12:20computer, or if you want to be able to code interactively or collaboratively

12:25with other people,

12:26right, you can actually work on the same Python code at the same time, which

12:30can be great for

12:31things like tutoring or, you know, doing a hackathon, right, if you're, if you

12:36're working on this with

12:37a friend. But anyway, the way that this works is pretty simple. It's, it's

12:41pretty much like an

12:42online version of visual studio code that we've been working with here, right,

12:46just like how visual

12:47studio code has a space for you to write code in files, and a space for you to

12:53work with the

12:53terminal. Over here in replit.com, you have your files over here, and you can

12:59create new files just

13:00by creating them over here on the left hand side. And you can also run that

13:06code by clicking this

13:07run button. So again, if we do x equals, let's do 10 this time, and maybe y

13:11equals 20. And then

13:14you say print x plus y. Well, if you click this run button, what that's going

13:18to do is run your

13:19code, and print out the output right here in the console. All right, so that's,

13:24um, again, this is

13:26just a really nice way to work with Python without having to worry too much

13:30about get, about getting

13:32things set up. So if you want to follow along in this instead of with visual

13:36studio code, feel free

13:38to do that. It'll work at least while we're learning the basic syntax for, you

13:43know, as just a basic

13:44way to follow along. So anyway, those are three basic ways of writing and

13:49running Python code.

13:51We're going to be using pretty much all of these at some point in the, in the

13:56skills of this course.

13:57So what I'd like you to do right now is just take a few minutes and play around

14:01with these

14:02right simple programs experiment around with just the, even just the simple few

14:06things that we've

14:06learned so far, defining variables and printing things out, just, just so that

14:11you get a feel of

14:12doing it and can kind of do it by heart, right? That'll, uh, that'll make

14:15everything that comes

14:16after this a lot easier for you. So anyway, I hope this has been informative

14:21for you, and I'd

14:22like to thank you for viewing.

0:00All right, so now that we've seen the basics of writing and running Python

0:04programs in

0:05a few different contexts, let's move on now to talk about the basics of Python

0:11syntax.

0:12Right now, we're going to be learning about Python syntax in quite a bit of

0:15detail.

0:16We're going to go into a lot of depth in different parts of Python's syntax.

0:21But what I'd like to do here before we get into all of those details is take a

0:26look at

0:26just some of the basic concepts of Python syntax.

0:31So the first thing to know is that in Python, instead of using things like

0:38curly braces,

0:40right, so in JavaScript, for example, with curly braces, what you would usually

0:45do is

0:46use them to define blocks of code.

0:48We use these to group pieces of code into things like functions or classes,

0:53things like that.

0:55So in JavaScript, if we were to create a function, which is just a reusable

0:59piece of functionality,

1:01we'll talk about functions in Python in more detail shortly.

1:04But if we were to create a function called something like my function in

1:08JavaScript, we

1:08would actually use curly braces to sort of denote where the body of that

1:14function starts

1:16and ends.

1:17However, in Python, you're not really going to see too many of these, at least

1:23not used

1:23in that context.

1:25In Python, what you'll see is that almost always indentation is used to group

1:32code.

1:33Now what this means is that unlike in many other programming languages with

1:38Python, you

1:39can't just indent things when you want to, right?

1:42So if we were to indent this print x plus y thing, you'll see right away that

1:46in the

1:47IDE, it recognizes that there's something not quite right with that syntax.

1:52And if you try and run this program, I will say Python three, my first program

1:57dot pi

1:57again, you'll see that instead of giving us the instead of giving us x plus y,

2:02which

2:03is 11 in the console, it says indentation error, unexpected indent.

2:07And it even shows us the line here.

2:09And then by the way, this is a, this is an error in Python when something bad

2:14happens,

2:15either, you know, whether that's something that we've written that doesn't work

2:19, or

2:19whether that's something in the code that breaks, you're going to see it

2:23printed out

2:24to the console like this.

2:27And usually it'll come along with a nice little, a nice little description

2:30describing

2:31where the error occurred, and what the problem is, right?

2:35So in this case, we see unexpected indent again, basically just telling us that

2:39we're

2:39not supposed to have that unexpected indent.

2:42So in Python, right, if we were to define a function, and again, we'll talk

2:46about this

2:47in more detail later, the way that we do that is by creating the function name.

2:52All right, so we could say something like define my function.

2:56And then the body of the function is simply going to be underneath that with a

3:01single

3:01indentation before each line, right?

3:05So you know, we could say x equals one, y equals two, z equals three, right?

3:11All of those are going to start a little bit with a little bit of an indent

3:16ation past

3:16the function name itself.

3:19And again, this is how this is how code, right, different lines of code that

3:23are related

3:24to one another are grouped together in Python.

3:28We don't use curly braces or other symbols like that to do the job.

3:33All right, so we'll talk about the indentation rules in much more detail later

3:37as well.

3:38But that's just a main concept that I wanted you to understand in Python is

3:43that indentation

3:44is very important for grouping your code together.

3:48All right, so besides that, in Python, a few other syntax related things that I

3:55wanted

3:56to mention is any line that you write, such as this x equals five, y equals six

4:02, print

4:02x, y thing, is by default, when we run our code, the Python interpreter is

4:07going to try

4:08and execute that.

4:09So if we write something that isn't valid Python syntax, right, if we write

4:14something

4:14like hello, Sean, right, something like that, well, that's not valid Python

4:20syntax.

4:21And if we try and run our code with that in it, it's going to say, yeah, I

4:24really don't

4:25know what that is.

4:26It's going to give us a syntax error in this case saying invalid syntax.

4:30However, in many cases, it might be nice for us to be able to, you know, write

4:35things like

4:36notes next to pieces of our code so that we can remember what they do.

4:41Right.

4:42In this case, with x and y, why are we setting x to five and why are we setting

4:46x to six?

4:47Well, it might be really nice to sort of leave a note for ourselves.

4:51Right.

4:52If we say something like x is the number of people invited to the party,

4:59something like

5:01that.

5:02Right.

5:03Again, because we've written this in the file, it's going to cause a syntax

5:08error if we were

5:09to try and run this, right.

5:10And I'll just show you that again.

5:12Yep.

5:13Sure enough, it doesn't really know what we're trying to do there because again

5:15, that's not

5:16valid Python syntax.

5:18So if we want to write notes next to our code, we can do that by using

5:23something called comments

5:25in Python.

5:27These are a very common thing that are used really no matter what programming

5:31language

5:31you're using.

5:33And comments in Python are indicated by using a number sign, right, or hashtag

5:37as the young

5:38folks these days are calling it.

5:41So by putting a number sign at some point in our Python program, we're saying

5:46that whatever

5:47comes after that number sign on the line, we want Python to completely ignore.

5:53Right.

5:54So Python's not going to try and run this now.

5:56And if you run the code, sure enough, what we'll see is that it, you know, it

6:01doesn't

6:02have a problem with that now.

6:03We could write really anything we want after this little number sign.

6:08And it's really for our eyes only the Python interpreter isn't going to look at

6:12it.

6:12All right.

6:13So we could say X is the number of people invited to the party.

6:16Maybe this is like why is the number of people who showed up uninvited.

6:24Oops, that's uninvited.

6:27There we go.

6:29And then you could say the total number of people at the party, right?

6:34And this, the, you know, the idea behind this is that these comments would make

6:40it much

6:41easier to remember what this code was supposed to do when we look back at it a

6:47day later

6:47or a week later or a month later.

6:49So especially as you're learning Python and getting comfortable with the Python

6:55syntax

6:55and writing your own Python programs, you're going to want to make liberal use

7:00of these

7:01comments because they'll allow you to, again, remember what was going on.

7:06So as we learn how to do things like declare variables, right, which we've done

7:09here with

7:10X and Y, you could say something like, uh, this is how to declare a variable,

7:16right?

7:17And that's all you need to do there to remember.

7:19So in this way, uh, I would highly recommend you do this as you're going

7:23through all of

7:24these videos.

7:25In this way, you can have kind of, uh, all of the example code that you write

7:29will end

7:29up being just, it will end up being sort of like a cheat sheet for you that you

7:34can use

7:34to review everything that you've learned so far.

7:38All right.

7:39Um, so anyway, that's really the, the main pieces of syntax that I wanted to

7:44cover in

7:44Python will cover a lot of other things such as how to declare variables, uh,

7:49simple,

7:49uh, arithmetic operations, declaring functions, things like that, uh, in later

7:55videos and

7:56skills, but that's, those are really the main two things that I wanted to, uh,

8:00show

8:01you here because I'll be using them quite a lot going forward.

8:05Uh, so just as a review here, the first thing is that indentation in Python is

8:10very important.

8:12It's used to group together pieces of code.

8:15Uh, so as we start seeing how to do things like loops or functions, things like

8:20that,

8:21we'll be using indentation and only indentation to group everything together.

8:26So that's really something that you've got to get used to in Python, which is

8:29why I wanted

8:30to mention it right up front here.

8:32And the second thing here is that you can create comments by using the little

8:38hashtag

8:38or number sign.

8:40So that's really all that I wanted to go over here, uh, in terms of general

8:46Python syntax.

8:47So I hope this has been informative for you and I'd like to thank you for

8:50viewing.

0:00Okay, so at this point we've seen the basics of running Python programs and we

0:04've also

0:05talked about just a few fundamental concepts in Python syntax.

0:10So let's move on and start taking a look at a few useful tasks.

0:15If you want to call them that, that you're going to be performing in any Python

0:20program

0:21that you write no matter what it is.

0:22All right, so what we're going to take a look at here is the basics of working

0:26with variables

0:27in Python.

0:29Now we've already seen how to declare variables when we said x equals five and

0:34y equals six.

0:35So essentially when you're working with variables in really any programming

0:41language, there

0:42are really two different ways that you might use them.

0:47One is basically you're just giving another name to a value.

0:54So you could say something like pi equals 3.14159.

1:00However far you want to go there, right?

1:03And in this case, the name pi is really just another name for this value here.

1:10So if you were to say, you know, if you were to use that later on, if you were

1:14to say things

1:14like print pi or if you were to multiply something by pi later on in your

1:20Python program, you

1:22could just use that name pi instead of having to type out this value each and

1:26every time.

1:27So that's really the first usage.

1:30And in recent years, this is actually an increasingly common usage of variables

1:38, even in situations

1:39where you might otherwise want to change things.

1:43Don't worry too much if that doesn't make sense.

1:44I'll discuss that in more detail later.

1:47But the second way that you might think about a variable is as sort of a

1:51container that

1:52might hold different values over the course of its lifetime.

1:56So, you know, if you say something like x equals 5 and then later on, you

2:01change the value

2:03of x.

2:04So you say x equals 10 and then maybe you add something to x by saying x equals

2:09x plus

2:10one, which would be incrementing x.

2:12Well, in this case, we can't really say that x is another name for any of these

2:18values,

2:19right?

2:20x doesn't mean 5 because later on in the program, x could mean 10 or 11 or

2:24really anything else.

2:26So that's kind of the other potential way of thinking about variables is as

2:31containers

2:32that can hold different values over their lifetime.

2:35And as you're going to see, you know, as we go through learning Python syntax,

2:39we're

2:40going to actually use both of these ways of thinking in our program.

2:45So these are really just tools that you can use, right?

2:48Ways of thinking that you can use in order to really understand the purpose of

2:53variables.

2:53Now, just as a sort of side note here, this first way, right, since the

3:00variable value

3:01doesn't actually change and therefore it's not truly a variable in the meaning

3:07of something

3:08that varies over time.

3:11This is sometimes referred to as a constant.

3:15So really a constant is just anything whose value doesn't change and therefore

3:19remains

3:20well constant.

3:22But Python, unlike a lot of other programming languages, doesn't actually have

3:27a, it doesn't

3:28actually have a keyword that will prevent you from changing the value of a

3:34variable.

3:35So if you want something to actually be constant, you have to be a little bit

3:38disciplined and

3:39not change it later on, right?

3:41Later on, if we were to say pi equals four, well, that's not really a constant

3:44anymore,

3:45right?

3:46And from that point on, pi would have a different value.

3:49So you might hear me use the word constant sometimes when talking about this

3:52way of doing

3:53things and variable when, you know, talking about this other way of doing

3:58things where

3:58the value actually changes.

4:01But really in Python, they're technically all variables because they can all

4:06change.

4:07So variables, we've seen, we've already seen how to create two variables here,

4:13but variables

4:14can store a pretty wide range of data.

4:17In fact, pretty much any thing that you can, you know, any type of data that

4:21you might

4:21have in a Python program, and we'll talk about these types in much more detail

4:26later

4:26on.

4:27I feel like I keep saying we'll talk about these later on and we will.

4:32But besides simple integer numbers, like five or six, right, counting numbers,

4:37again,

4:37we'll talk about integers and different number types later on.

4:41But besides those, you can also store things like text in variables.

4:46So if you wanted to say something like message equals hello, right, you can do

4:52that in Python.

4:53If you wanted to store something like just a yes or no value, well, that's

4:57something

4:57called a Boolean in Python.

4:59And again, we'll get into more detail on that later.

5:02But that might look something like this.

5:03We could say something like Sean likes coffee equals true.

5:10And in Python, by the way, in case you're already familiar with true and false

5:13values

5:13from another programming language in Python, there's, there's really only two

5:18values here

5:18true with a capital T or false with a capital F, right?

5:24Those those represent really yes and no, respectively.

5:28And besides this variables can also hold more complex types of data.

5:33And we'll cover these later on as well, but they can hold things like lists,

5:37which in

5:38Python are just ordered series of values.

5:42So we could say something like lucky numbers, something like that equals 123.

5:51And in this case, this variable is holding multiple values, right?

5:56So a variable again is really just kind of a label for some other value that we

6:03want

6:03to work with later on in our program.

6:06And we want to work with it by name instead of again, having to specify the

6:09value each

6:10and every time.

6:12So anyway, that's some some of the different data types that variables can

6:16store.

6:17And in case you're eager to know the names of these things, these up here are

6:21called

6:22numbers or more specifically integers in Python.

6:26We'll go into more detail on how those work later, as I've said, these are

6:30called strings.

6:31So really any piece of text in Python represented like this is called a string.

6:37These as I said are called booleans and they're just yes or no values, one or

6:42zero, right?

6:43And these are called lists.

6:45And these are just a few of the data types that you can use in Python.

6:49All right.

6:50But anyway, really, the only point that I want to make at this time is that

6:54variables

6:55can store pretty much any type of data that we want.

6:59And we can change it whenever we want, as I've said, right?

7:02So let's take a look at just one possible, very simple application that you

7:08might want

7:09to use this whole variable thing for.

7:13Let's say that we have a few numbers, which are called x, y and z.

7:16And you want to add all of them together, but you want to do it like one by one

7:20, right?

7:21So you want to add x, you want to add y, and then you want to add z.

7:24There are multiple situations where you might want to do this in Python.

7:28For example, if you want to add all of the numbers in a list, which is

7:31something again

7:32that we'll take a look at later.

7:35But in case you in case you don't know what that means, just follow along with

7:38me here

7:39for a second, we could say something like sum equals zero or total equals zero.

7:47And then we could just go through and add each of these variables one by one.

7:52So we could say total equals total plus x.

7:57And then total equals total plus y, and then total equals total plus z.

8:05And after we do that, we could just print out total and total would contain the

8:11sum

8:11of all of those values.

8:13So if we run our program here, let's just run Python my first program dot pi

8:18again, we'll

8:18see that that gives us 18.

8:21All right.

8:22And basically, in cases like this, all that happens, at least when you're

8:26working with

8:27simple data types, all that happens is Python basically goes through and

8:33replaces any instances

8:35of that name, right, that variable name with the current value of that variable

8:42.

8:42So when it sees total, it's going to look, okay, cool.

8:46What's the current value of total?

8:48Oh, it's zero.

8:49Okay.

8:50Well, that means zero.

8:51And then it comes over here to x.

8:52It says, okay, what's the current value of x?

8:54It looks to see that it's currently five.

8:57So it basically just kind of substitutes that in there.

9:01And that gives us zero plus five.

9:04This whole thing turns into the value five.

9:06And total is now equal to that, right?

9:09That's sort of the process that Python goes through when it's executing our

9:13lines of code.

9:14All right.

9:15So now that total is five here, and let's just delete that now, now that total

9:20is five,

9:21the next line, total again is going to be replaced with five.

9:25And it's going to look and see what y is.

9:28It's going to see that y is six.

9:29So it's going to turn that into five plus six, which is equal to 11.

9:33And total is now 11.

9:36And lastly, it's going to substitute 11 for total.

9:40It's going to substitute seven for z.

9:43And that ultimately gives us 18.

9:45So total is now 18, which means that when we go to print the total, we're

9:49really just

9:50saying print 18.

9:53Okay.

9:54Now, obviously variables are much more useful than what I'm making them out to

9:59be right

10:00here.

10:01Really, they what they allow us to do is work with data in a slightly more

10:06abstract way.

10:07Right?

10:08So it allows us to say things like, you know, a equals two B plus C or

10:14something like that.

10:17And we don't need to actually know ourselves, right?

10:21We don't need to actually know as humans what B is or what C is.

10:26We just know that A is two times B plus C. All right.

10:31That's really all that we need to know.

10:32And that's where, again, variables allow us to work with data in the abstract.

10:37You'll, I think you'll get a sense of this, a much deeper sense of this as we

10:41continue

10:42learning about Python.

10:43But anyway, that's the basics of declaring variables and just a few things that

10:49you can

10:49do with them.

10:51One thing that you'll notice in Python is that there are a few, there are

10:55actually a

10:56lot of shortcuts, right?

10:59That allow us to write less code and achieve the same thing.

11:02So as an example, if you look at these three lines here, you'll see that there

11:06's a little

11:07bit of repetition, right?

11:08We're saying total equals total plus X, total equals total plus Y, total equals

11:13total

11:13plus C, right?

11:14Well, it would be nice if there was a shorthand way that we could just say

11:18something like

11:19add X to total, not total equals total plus X, right?

11:23And as a matter of fact, there is this is something that you'll see very

11:26commonly used

11:27in Python.

11:29In fact, you'll almost never see or you'll very rarely see what I wrote here.

11:34What you'll see instead is total plus equals X, all right?

11:40And total plus equals Y and total plus equals Z.

11:46And again, this basically means add X to whatever the value of total is.

11:52You might also think of this as saying something like increment total by X or

11:55increment total

11:56by Y.

11:57So I hope this has been informative for you and I'd like to thank you for

12:00viewing.

12:01[BLANK_AUDIO]

0:00All right, so now that we've taken a look at the basics of declaring variables

0:04in Python,

0:05the next thing that I'd like to talk about before I give you a challenge to

0:09kind of sum

0:10up what we've learned so far, the next thing that I want to talk about is the

0:15rules around

0:16naming variables in Python.

0:19So so far, the variable names that we've used have been pretty straightforward,

0:23right?

0:23We have x, y, z and total.

0:26So in many cases, you're actually going to want to have your variables be much

0:30more descriptive

0:32than this.

0:33Frankly, just because it makes it easier to read your code when your variable

0:38names are,

0:40you know, when your variable names are much more descriptive again, picture

0:42that we were

0:43to say something like a equals x plus y plus z and compare that to something

0:51like total

0:52people at party, something like that equals people or invited people, you could

1:01even just

1:02say invited plus uninvited plus my family, something like that, right?

1:11That's a lot easier to tell what's going on in that, you know, in that line of

1:16code than

1:16if we were to just name our variables rather obscure, you know, letters or

1:24things like

1:25that.

1:26So keeping in mind that our variable names are often going to be a little bit

1:30more complex,

1:31sort of like what I just had here.

1:33Let's talk about some of the rules around naming variables.

1:36The first rule around naming variables is that they need to start with a letter

1:43or the

1:44underscore character.

1:46So we'll just write start with and then we'll say letter or the underscore

1:55character, right?

1:57That's what that is there.

1:58You just you hold down shift and type a dash character and that's what you get.

2:03So that's the first rule.

2:04And what this means is that you can't have variables that start with that start

2:08with

2:09numbers, right?

2:10That just doesn't make sense.

2:11And that interferes with Python's syntax, which is why it's not allowed.

2:16The second rule here is that the rest of the characters in the variable name

2:22can be either

2:23letters.

2:24So we'll say rest can be letters.

2:31They can be numbers.

2:32So you can include numbers in your variable names.

2:37So if you wanted to say person one or person two or something like that, you

2:44can.

2:44And the underscore is allowed as well.

2:47All right.

2:48So basically what that means is that variable names can consist of letters,

2:54numbers and

2:55underscores, but they can't start with a number.

2:58I suppose you could kind of combine those two rules into one.

3:00But anyway, the third rule is that variable names are case sensitive.

3:06So if you were to write something like, you know, person one, all in lower case

3:12, that's

3:12not going to be the same thing as if you were to write person one all in upper

3:16case, right?

3:17That's Python just isn't going to consider those to be the same thing.

3:23And a fourth one, just in case that wasn't obvious from the second rule here is

3:28that spaces

3:29are not allowed in Python variable names, right?

3:33So Python would think that those were supposed to be two separate variables or

3:37something

3:38like that.

3:39All right.

3:40And one last rule here is that you can't pick a variable name that is already a

3:46reserved

3:47keyword in Python.

3:49Now we haven't talked about reserved keywords in Python, but we actually have

3:52seen two of

3:53them.

3:54And that is when we, you know, when we were talking about how variables work,

3:58we saw

3:59that with Boolean variables, which are yes or no values, there were two values

4:04true with

4:05a capital T and false with a capital F. So as you might have guessed, you can't

4:11have

4:11a variable name with either of these names.

4:15You can't have a variable with either of these names because Python would get

4:20very confused,

4:21right?

4:22So we'll say no reserved keywords.

4:24All right, and that would again be things like true or false.

4:31So anyway, those are the main rules behind creating some variable names.

4:37All right.

4:38And I suppose let's just, I'll just go through a few examples of what are valid

4:42and what

4:43are not valid variable names according to these rules.

4:46And feel free to take a screenshot of this if you want.

4:49But anyway, as I said, the first example that I'll give you will be a valid

4:55variable name

4:56or you know what I'll start off by giving you an invalid variable name, an

4:59invalid variable

5:00name would look something like this 50% off, right?

5:05If we were, if we were to try and have a variable like that, that expressed

5:09whether something

5:10was on sale for 50% off, we're not allowed to have that as a variable name

5:14because it

5:15starts with a number.

5:17So in this case, you would actually have to say, you know, you'd actually have

5:20to type

5:21out the word 50.

5:23Or if you wanted to, you could fix it by putting a underscore before it, right?

5:29That would be allowed.

5:30Or you could say percent off 50, right?

5:35And you could, you know, you could put the 50 at the end.

5:38It just can't be at the beginning.

5:40So that's the first example I'll give you.

5:43And in fact, let me just show you this, we can actually use comments here to

5:47keep notes

5:48of this so we can keep that there and we can say no.

5:53And then we can create a better version of it where we say something like 50%

6:00off and

6:01that is allowed, right?

6:04So next example that I'll give you is something to do with the subsequent

6:08characters, right?

6:09Something except for the first character in a variable name.

6:14And that might be something like, you know, if you want to say user name, well,

6:18you might

6:18want to use, you know, you might want to separate those two words using a dash.

6:23And that's not going to work either.

6:24Here, let's just give it a value here, Sean.

6:28That's not going to work either because dashes are not allowed in words and in

6:33variable names

6:34rather.

6:35The question is that to Python, this looks like user minus name, right?

6:41So that just isn't going to work in this case.

6:44Dashes really aren't allowed.

6:46In fact, no symbol is allowed in variable names except for that underscore

6:51character.

6:51So we would have to change that to user underscore name if we wanted to do that

6:57.

6:57So let's just, let's just add that dash back in.

7:00I'm going to comment that out and I'm going to add no at the end of it.

7:04Like so.

7:05And then we'll add a fixed version underneath this.

7:07You can use this as notes if you want.

7:09We'll say username equals Sean.

7:11All right.

7:13So another example here, let's take a look at case sensitivity.

7:16Let's say that we were going to say something like print total.

7:19If we wanted to print the total out up here, well, that's not going to work

7:22because we

7:23don't have any variable called total all in caps like that, right?

7:28So that's not going to work.

7:29I'll just comment that out so that that doesn't, that doesn't stop our program.

7:33And by the way, the reason I'm commenting these out is because if we were to

7:36try and

7:37run it without them commented, we wouldn't get past that line, right?

7:41Python would run into this.

7:42It would produce an error and it wouldn't, you know, we wouldn't get to see the

7:47output

7:48of anything that came after that.

7:50So anyway, we could of course print the total.

7:54In fact, we've already done so up above here.

7:56So I'll just, in fact, just move that down there.

7:58All right.

7:59Cool.

8:00So next thing, no spaces.

8:03You know, obviously we can't have something like my favorite color equals, and

8:09then, you

8:10know, let's just say blue, you can't have that, right?

8:14Python has no idea what's going on there.

8:15It thinks that we're having, you know, things that one of these is supposed to

8:18be a keyword

8:19or something.

8:20It just doesn't like that at all.

8:22So we'll put no next to that as well.

8:25And that would have to be something like this, where we actually use

8:28underscores, and this

8:29is a very common usage of underscore characters in Python is to basically serve

8:36as spaces

8:36in the word.

8:38And by the way, you might also see in Python variable names that instead of

8:43using underscores

8:45to separate words, you might see something like this, where someone starts the

8:50first

8:51letter of each new word with a capital letter.

8:57And this is something that's referred to as camel case, right?

9:01The reason being that these look like camel humps apparently.

9:04Someone came along and thought that and well, the name stuck this rather

9:08fittingly considering

9:10the name of the program and language we're learning is called snake case, right

9:14?

9:15Because apparently someone thought that this looks like a snake somehow.

9:18Anyway, those are the names for those two different types of casing snake case

9:23in Python

9:24is usually used more often.

9:26You might see that someone coming from another language such as JavaScript will

9:30use camel

9:30case for, you know, for file names, but snake case is far more common from what

9:37I've seen.

9:38All right, so one last one last example that I'll give you fitting in with the

9:44fifth rule

9:44is if we were to try and say something like true equals, you know, true, maybe

9:49we wanted

9:50a variable called true that held the string true.

9:53Well, we obviously can't do that because Python thinks that this is a reserved

9:57keyword.

9:58And in fact, it is a reserved Python keyword.

10:00So we can't do that either.

10:02Let's just write no there like so.

10:05And you know, if we wanted to do something else like that, we could in theory

10:09use true

10:09or lowercase like that.

10:13But usually with with keywords like that, it's better to just find another, you

10:17know,

10:18it's better to just find another way to express that, right?

10:22You could say something like is true or true string or true text, something

10:28like that.

10:29Any of those would be better than just saying true equals true, right?

10:34One last thing that I wanted to point out here too is that aside from reserved

10:39keywords

10:39such as this true thing here in Python, and there are many other reserved

10:44keywords that

10:45we'll talk about later on in other skills, you might also come across things

10:50that aren't

10:52aren't actually reserved keywords, but are already something else.

10:55So this print function, for example, this already, you know, the word print

11:01already means

11:02something in our program just by default in Python.

11:05So we wouldn't want to say, you know, print equals hello, right?

11:11And the problem with this is we could do that.

11:14And we'll let us do this.

11:16But what that does is it gets rid of the default functionality of print, and it

11:21makes print

11:22just in this case, it makes it just a piece of text, right, a variable with

11:26some text

11:26inside of it, which is generally not what we want.

11:29And another instance of this, which, you know, especially in beginner

11:33programming exercises,

11:35you will see, you know, you'll be very tempted to use this is some equals zero

11:40sum is actually

11:42already a function in Python that you can use to get, well, the sum of a series

11:48of numbers.

11:49We'll talk about this in more detail later, but you don't want to do either of

11:52these things.

11:53So let's just comment those out.

11:54And we'll say this works, but overrides the print function.

12:06And then I'll say something like same here, some is a built in function.

12:11All right, so you don't want to do either of those things.

12:13You'd have to just find another word for it like we did up here, right?

12:17So total, you know, or for print, you could say something like message equals

12:22hello, right?

12:23That would just be a better way to do that.

12:26So we'll say total equals, but I won't use total now.

12:29I'll use something like count equals zero, right?

12:33So anyway, that is the basics of naming conventions and naming rules in Python.

12:39So I hope this has been informative for you,

12:41and I'd like to thank you for viewing.

0:00All right, so at this point we've talked about some of the basics of Python

0:03syntax and we've

0:04also learned some details about Python variables. So it's time for you to solid

0:10ify what you've

0:11learned so far by doing a challenge. Now in this challenge, what we're going to

0:17do is I'm going to

0:17give you a few different situations that can be expressed by using variables

0:23and your job is going

0:24to be to write that situation as a Python program. So to give you an example

0:32here, let's say that

0:33I give you the situation of calculating how much of a tip you want to leave at

0:40a restaurant and

0:41using that to calculate the total bill. Well, if we think about what that would

0:47look like here,

0:48right, there are really going to be two main quantities involved. The first is

0:56the bill amount,

0:57right? So let's say that you go to a restaurant, you spend maybe $40 just to

1:03give you an example

1:04here. So let's try that again. There we go. And let's say that you want to

1:09leave a 15% tip,

1:11right? Well, so we'll just write that as tip percentage. And notice here that I

1:16'm not using

1:17variables yet. I'm just kind of drawing this out. And that can be very helpful

1:21sometimes to,

1:22you know, just to kind of work out your thoughts and get them more clear so

1:27that you can actually

1:28express them in code. So let's say that we want a 15% tip. And a few other

1:34things that we might

1:35know about this would be that the tip itself, right, is the way that you

1:41calculate the tip is by

1:43multiplying the bill amount times the tip percentage, whatever that number is,

1:51divided by 100, right?

1:53So we'll say tip percentage divided by 100. And that'll give you the tip amount

2:01, right?

2:02And finally, we would know that the bill amount, right, that the total amount,

2:07the bill plus the tip

2:09would be equal to the bill amount, plus the tip amount. All right. So those are

2:21just a few things

2:22that we know. And if you wanted to express this as a program in Python, here's

2:28what this would look

2:29like really the main purpose here is just to get you used to taking situations

2:34and mapping them into

2:36a fairly simple series of variables. So here's what that would look like here.

2:41What I'm going to do

2:42is create a new file, we'll call this something like variable challenge dot pi.

2:47And let's start off

2:50here by defining those things that we had previously as variables. So we would

2:57say something like

2:59bill amount, that would be a variable. And maybe that's $40. We'd have the tip

3:04percentage, which would

3:06be 15. All right, 15%. And from there, we can pretty much figure out the rest

3:14just by defining

3:15variables as combinations of these. So as I said, we could have the tip amount

3:23equal to the bill

3:25amount, plus, or sorry, times, and then we have the tip percentage divided by

3:34100. All right.

3:36So we now have the tip amount, which means that we can figure out the total

3:41bill. So we'll say total

3:43bill equals bill amount plus tip amount. All right. And what you can see there

3:52is if you print this

3:53out, right, we can say print, and we can print out any of this that we want. So

3:57if we want to see

3:58the tip amount, we can say something like print, well, here, we'll say the tip

4:03is, and then we'll

4:05print out the tip amount underneath that I'll show you how to actually add

4:11values into strings a

4:12little bit later. But then we'll say the total bill is, and then we'll print

4:20out the total bill

4:22underneath that. All right, so I'm going to run this now and what this will

4:25look like. Oops,

4:26let's rephrase. All right, so I'm going to run this now and what this will look

4:29like as we'll say

4:30Python three variable challenge dot pi. And sure enough, we see that the tip is

4:36$6. And the total

4:38bill is $46. So the cool part about this now, and you know, the reason that

4:43expressing

4:44situations in code like this is so helpful is we can now go up and change

4:51either of these two

4:52variables at the top. And everything else will still work, right? So let's say

4:56that we go to a

4:57restaurant now and we spend $100 and we want to tip 18%. Well, that's still the

5:04rest of the

5:04program is still going to work in exactly the same way. In other words, if we

5:08make a line of,

5:09so let's try that again, if we make a line of comment signs there just to kind

5:13of create a

5:14visual separation, we can we can basically change the entire rephrase. We can

5:22use this program in

5:23basically an infinite number of ways just by changing this stuff above the line

5:29. We don't

5:30really need to change anything down below. Unless of course we want to change

5:35the program

5:36significantly, right? So if we wanted to start calculating tax and add that in,

5:40well, we would

5:41have to add a tax percentage or something like that to this, maybe that's 4% or

5:47something,

5:47and we would have to incorporate that into the into this equation, right, this

5:53system of equations

5:55down at the bottom. But I'll just leave that off for now. And let's just run

5:58this again. And

5:59sure enough, we see that this works for those, you know, those modified numbers

6:03as well.

6:03So anyway, that's that's kind of the basic idea. So now that I've given you a

6:09demonstration,

6:10it's time for you to do a few of your own based on situations that I'll

6:14describe for you. All

6:16right, the first one, which should be pretty straightforward because it's

6:19pretty similar to

6:20the code that we just wrote, I'll give you that hint, is calculating the

6:25discount price.

6:27All right, there we go. Based on the original price and the discount percentage

6:35, right, so those

6:38original price and the discount percentage are going to be the two variables

6:41there.

6:41Another situation that you can do here is doing something like calculating the

6:48travel time.

6:49And this one would have probably two variables as well, the distance and the

6:57average speed. Okay,

6:59so again, you can express that as a as a program as well. And finally, I'll

7:05give you one more.

7:06Let's say that you want to calculate the total, your net calories for the day,

7:12right, the number

7:13of calories above or below zero based on diet and exercise. And in this one,

7:20what you could do is

7:21you could have like four variables, let's say you could have one variable for

7:26the calories for

7:27breakfast, one for lunch, one for dinner, and you could have a fourth variable

7:31for the number

7:32of calories you burned while exercising today. All right, so that would be,

7:37that's another program

7:38that you can feel free to work out on your own as well. Again, this is all just

7:43to get the,

7:43just to get the feel for defining variables and using them to express programs.

7:50So anyway,

7:51this is your challenge here. Feel free to give this as much time as you need,

7:55probably about five

7:56to 10 minutes. And once you rephrase. And once you've given this a try, feel

8:03free to move on to the

8:04next video where I'll walk you through the solutions. So I hope this has been

8:08informative for you. And

8:10I'd like to thank you for viewing.

0:00All right, well hopefully you've given this challenge a try.

0:03Let me walk you through these solutions.

0:06So the first project or mini program

0:10that I gave you as the challenge,

0:11and I'm just gonna open up the variable challenge here.

0:14And in fact, I'll use the comment marks

0:17to kind of separate the first challenge

0:19or the demonstration that I did from the first challenge.

0:23This was to calculate the discount price of something.

0:29It's something in a store given the price

0:31and the discount percentage.

0:33Well, this was going to be pretty much like the tip calculator,

0:37except a little bit in reverse, right?

0:39We'd be subtracting the discount price

0:42instead of adding the tip.

0:44But here's what this looked like.

0:46This required two variables, the original price.

0:51All right, which you could have set to anything, right?

0:53You could have said maybe 199 as the price.

0:56And then for the discount percentage,

1:01which is the other variable that we need,

1:04let's just say maybe 35%.

1:06All right, so now we just need to kind of figure out

1:09what the basic equation is for getting the discount price.

1:14And here's what that would look like.

1:17We'll say discount price.

1:18And as I said, this is gonna be very similar

1:21to the tip calculator.

1:22We're just gonna say discount price equals original price minus.

1:27And then what we're gonna do is we're going to figure out

1:31how much we want to subtract.

1:34So if we're removing 35% of the price,

1:37what that means is we need to multiply the price,

1:41or multiply the original price by 35,

1:46or we could maybe subtract 35 from 100

1:50if we wanted to do it the other way.

1:51But what this is gonna look like,

1:53'cause I'm just gonna say original price minus discount percentage.

1:58All right, that's gonna be divided by 100

2:01because we need to actually get that into the right form.

2:04And then we're gonna multiply that by the original price.

2:07So this is removing 35% of the price.

2:09There are other ways you could have done this

2:11or slight differences in the equation that you could have done,

2:14and that's fine too.

2:15So what I'm gonna do is I'm just going to say print,

2:19and then we'll print out the discount price is,

2:24and then we'll print out the discount price underneath.

2:27So let's say discount price.

2:29And if we run this code now,

2:31let's try running Python3 variable challenge.py.

2:35Sure enough we see that the discount price is $65

2:38and around 64 cents.

2:40Don't worry about that long string of decimal digits.

2:44We'll see how to format strings and numbers

2:47a little bit later in our Python journey here.

2:50Anyway, that was the first solution.

2:53So the second one was calculating the travel time.

2:57So there were two variables for this one as well.

3:00The first was going to be the distance,

3:03and you can say this in miles or kilometers, whatever you want.

3:07I'm gonna say that it's in miles.

3:09So we'll say that the total distance is equal to maybe 100 miles.

3:15And then we'll say the average speed.

3:18We'll say that that's maybe 60 miles per hour.

3:22So now in order to figure out the travel time,

3:24all we have to do is say travel time equals distance

3:29divided by average speed.

3:33And that would give us the travel time.

3:34So we can print that out now by saying print.

3:37The travel time is, and then we'll say print travel time.

3:44And that should work.

3:46So let's try running this again.

3:47And sure enough, it looks like the travel time

3:49is around one and two thirds of an hour.

3:53And that looks about right to me.

3:54So let's do the third and final challenge,

3:57which is calculating the net calories for the day.

4:02Basically the calories you consumed

4:03minus the calories you burned through exercise.

4:06So let's just assume that we have all of these numbers ready.

4:10The first one would be breakfast calories,

4:13as I said in the challenge video.

4:15So we'll say that maybe you consumed

4:171000 calories for breakfast.

4:19Maybe you had a big donut or something like that.

4:21We'll have lunch calories.

4:22Maybe we were a little bit more well behaved

4:25for lunch and ate 400 calories.

4:28And then for dinner, we'll say dinner calories.

4:32And maybe we went back to our old breakfast habits

4:35and had 1000 calories, all right?

4:37So there we go.

4:38We have our three variables for calories consumed.

4:42So let's do exercise calories burned, we'll call them.

4:47And maybe we, I don't know,

4:50maybe we went for a two mile run in the sun

4:53carrying 100 pounds on your back, something like that.

4:58We'll say that this was maybe we burned 1500 calories.

5:01So now the net calories,

5:04this is a pretty easy equation here.

5:06The net calories would just be breakfast calories

5:09plus lunch calories.

5:11All right, so we'll add those on like so,

5:14plus dinner calories.

5:16And then minus exercise calories burned.

5:21All right, so that should be all we need.

5:23So let's just print out the net calories now.

5:26We'll say net calories and we'll say print net calories.

5:31And if we run our code now, we should see

5:33that sure enough we have 900 net calories.

5:36So we have a little bit more exercising to do, I suppose,

5:39unless well, it depends obviously

5:40on how many calories you burn just sitting around as I do.

5:44But anyway, those are the solutions

5:46for the challenge that I gave you, right?

5:49The three sort of mini challenges.

5:51So I hope this has been informative for you

5:53and I'd like to thank you for viewing.