We just saw how to pass a script to the Python interpreter.

Another way to use the interpreter is in a more exploratory interactive way.

You can actually open up a prompt that allows you to type Python code

line by line.

This is super handy when you just wanna see what some code does, and

not actually go and create a whole script.

This type of interactive programming prompt is pretty popular in other

languages.

Generically this this type of exploratory prompt is referred to as a REPL or

R-E-P-L.

which stands for Read, Evaluate, Print, Loop, which is basically what its role is.

It reads the line, it evaluates it, it prints the result and

then it loops back so you can add another line of code.

Python's REPL is often referred to as the Python shell.

It is a wonderful place to hang out as you are learning,

so I definitely want you to be familiar with it.

Come on, let's go explore.

So, to open up the REPL we simply type Python.

And we'll get some information about the version of Python that we're running.

So we're running 3.6.4 on Linux.

Remember, your workspace is running a Linux OS, or operating system.

These three greater than signs, or chevrons, as they're sometimes called,

are communicating that this is the place where we can write some code.

So, let's do it.

Since we know we have a working program up here, let's just write that out ourselves.

So, we want to make sure that we type it exactly like it is above.

Make sure that you keep everything lowercase, case matters in Python.

So, this prinT is different than the lowercase print.

So, that's the name of the function that we want to call, and

you call a function using parenthesis.

And now we wanna pass in a string.

And we can create a string using quotation marks.

We'll do that, open that up.

And then our characters, which here happens to be, Hello, World.

And then we close our string with another quotation mark.

And then, finally, we close our function call with a closing paren.

So let's go ahead and run that.

Awesome, Hello, World.

Now, a great thing about the shell is that it keeps it's history.

If you press the up arrow, you can get the last line that you just typed back.

This is handy if you had made a typo or

you wanna write a line that is very similar.

Like for instant, let's just say hello to you.

So I'm gonna put my name in here.

Hello, Craig.

Awesome, and Python shell is also a pretty good calculator.

You can use it to do math-like things, things like 1 + 2.

And so you'll see here that the result printed out 3.

Note that we didn't actually call the print function.

Now this is a good example of the REPL in action.

What happened was it read the line, 1 + 2, it evaluated it, what's 1 plus 2?

And then it printed the result, 3,and it looped back to the prompt.

It showed the result so that we could see it.

We'll get to some more math in the course in just a bit.

Now another thing that is wonderful about the Python prompt

is that you can help when you need it.

So, for instance, if we wanted to know more about the print function,

we could just call the help function.

So we say help and we pass in the function that we're interested in,

we're interested in print.

Let's see what happens.

So this kicks open the documentation for the print function.

Now, this is gonna have some terminology in here that we haven't yet

covered, so don't let that overwhelm you.

Now believe it or not, if you stick with it and

immerse yourself, this will all make sense.

We'll get to all of this, just not right now.

So here's the description, it prints the values to a stream, or

sys.stdout by default.

Standard out is another way of saying the place that you ran the program from,

the default output.

So basically, what this is saying is that we can print to other places, too.

This value here is the hello world string that we passed in.

Then you'll notice that there's a comma and then there's these ellipses,

there's these three ellipses here, right?

So, this means that we can actually pass multiple values to print, and

we'll do that here in a bit.

We'll pass multiple values.

Now, if you look down at the bottom here you'll see that that says END and that's

because we're inside of what is known as a pager, when help opens it does that.

Now, it just so happens that all the help fits on one page but let's go ahead and

let's make that not happen.

So this is something you can do too, you can make the console bigger or smaller, so

I'm going to make it smaller.

And you'll see what happens is, eventually,

there's this like blinking colon.

And I wanted to show you this just in case you opened up help to something else.

You can press the up and down keys and move around.

And space bar actually moves a page at a time.

So, to get out of a pager, what you can do is press Q.

And now normally what would happen when you popped out of here is you would pop

back into your shell.

But it looks like I had a little bit of a workspace problem.

Which I'm glad happened, so I can show you what to do when this happens.

So the console,

you can restart your console always by clicking either this X here.

This will close the console.

And I can up here and say View > Show Console.

It should pop back open, there we go.

And so if we are inside of a shell,

You might wanna know how to get out of here too, right?

So, there is a handy function called exit, and that will pop you out.

But even better, and I know cuz we programmers are lazy,

you can also press Ctrl + D to drop out of there.

There we go.

Awesome, so now you have a place to go and explore when you need to.

Also, you can't break anything in there.

So, feel free to do whatever.

And like you just saw, you can always exit out.

A common thing that happens to people just beginning to learn to code

is that they're afraid to make mistakes, so they freeze up.

Please don't be afraid of making mistakes, that will bite you in the future.

Try to change your point of view to this one, making mistakes is awesome.

Messing up simply means that you're trying, and

you can't learn without trying, can you?

So, don't let those mistake get you down.

It's part of the learning process, and REPL is awesome for

that kind of exploration.

I do get it, though.

Those error messages can be intimidating.

So let's do this, let's take a quick break, and

then take a look at some of the more common errors, and how to handle them.