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The Python Shell
6:06Learn how to use the Python interpreter interactively
We just saw how to pass a script
to the Python interpreter.
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Another way to use the interpreter is
in a more exploratory interactive way.
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You can actually open up a prompt
that allows you to type Python code
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line by line.
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This is super handy when you just
wanna see what some code does, and
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not actually go and create a whole script.
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This type of interactive programming
prompt is pretty popular in a lot of other
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languages.
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Generically, this type of exploratory
prompt is referred to as a REPL or
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R-E-P-L.
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which stands for Read, Evaluate, Print,
Loop, which is basically what its role is.
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It reads the line, it evaluates it,
it prints the result and
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then it loops back so
you can add another line of code.
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Python's REPL is often referred
to as the Python shell.
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It is a wonderful place to
hang out as you are learning,
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so I definitely want you
to be familiar with it.
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Come on, let's go explore.
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So, to open up the REPL
we simply type Python.
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And we'll get some information about
the version of Python that we're running.
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So we're running 3.6.4 on Linux.
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Remember, your workspace is running
a Linux OS, or operating system.
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These three greater than signs, or
chevrons, as they're sometimes called,
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are communicating to us that this is
the place where we can write some code.
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So, let's do it.
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Since we know we have a working program up
here, let's just write that out ourselves.
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So, we want to make sure that we
type it exactly like it is above.
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Make sure that you keep everything
lowercase, case matters in Python.
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So, this prinT is different
than the lowercase print.
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So, that's the name of the function
that we want to call, and
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you call a function using parenthesis.
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And now we wanna pass in a string.
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And we can create a string
using quotation marks.
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We'll do that, open that up.
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And then our characters,
which here happens to be, Hello, World.
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And then we close our string
with another quotation mark.
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And then, finally, we close our
function call with a closing paren.
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So let's go ahead and run that.
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Awesome, Hello, World.
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Now, a great thing about the shell
is that it keeps it's history.
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If you press the up arrow, you can get
the last line that you just typed back.
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This is handy if you had made a typo or
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you wanna write a line
that is very similar.
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Like for instant,
let's just say hello to you.
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So I'm gonna put my name in here.
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Hello, Craig.
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Awesome, and Python shell is
also a pretty good calculator.
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You can use it to do math-like things,
things like 1 + 2.
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And so you'll see here that
the result printed out 3.
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Note that we didn't actually
call the print function.
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Now this is a good example
of the REPL in action.
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What happened was it read the line,
1 + 2, it evaluated it, what's 1 plus 2?
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And then it printed the result,
3,and it looped back to the prompt.
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It showed the result so
that we could see it.
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We'll get to some more math
in the course in just a bit.
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Now another thing that is
wonderful about the Python prompt
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is that you can help when you need it.
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So, for instance, if we wanted to
know more about the print function,
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we could just call the help function.
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So we say help and we pass in
the function that we're interested in,
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we're interested in print.
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Let's see what happens.
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So this kicks open the documentation for
the print function.
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Now, this is gonna have some
terminology in here that we haven't yet
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covered, so don't let that overwhelm you.
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Now believe it or not,
if you stick with it and
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immerse yourself,
this will all make sense.
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We'll get to all of this,
just not right now.
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So here's the description,
it prints the values to a stream, or
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sys.stdout by default.
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Standard out is another way of saying
the place that you ran the program from,
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the default output.
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So basically, what this is saying is
that we can print to other places, too.
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This value here is the hello
world string that we passed in.
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Then you'll notice that there's a comma
and then there's these ellipses,
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there's these three ellipses here, right?
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So, this means that we can actually
pass multiple values to print, and
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we'll do that here in a bit.
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We'll pass multiple values.
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Now, if you look down at the bottom here
you'll see that that says END and that's
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because we're inside of what is known as
a pager, when help opens it does that.
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Now, it just so happens that all the help
fits on one page but let's go ahead and
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let's make that not happen.
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So this is something you can do too, you
can make the console bigger or smaller, so
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I'm going to make it smaller.
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And you'll see what happens is,
eventually,
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there's this like blinking colon.
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And I wanted to show you this just in case
you opened up help to something else.
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You can press the up and
down keys and move around.
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And space bar actually
moves a page at a time.
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So, to get out of a pager,
what you can do is press Q.
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And now normally what would happen when
you popped out of here is you would pop
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back into your shell.
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But it looks like I had a little
bit of a workspace problem.
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Which I'm glad happened, so I can show
you what to do when this happens.
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So the console,
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you can restart your console always
by clicking either this X here.
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This will close the console.
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And I can up here and
say View > Show Console.
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It should pop back open, there we go.
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And so if we are inside of a shell,
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You might wanna know how to
get out of here too, right?
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So, there is a handy function called exit,
and that will pop you out.
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But even better, and
I know cuz we programmers are lazy,
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you can also press Ctrl +
D to drop out of there.
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There we go.
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Awesome, so now you have a place to go and
explore when you need to.
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Also, you can't break anything in there.
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So, feel free to do whatever.
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And like you just saw,
you can always exit out.
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A common thing that happens to people
just beginning to learn to code
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is that they're afraid to make mistakes,
so they freeze up.
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Please don't be afraid of making mistakes,
that will bite you in the future.
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Try to change your point of view to
this one, making mistakes is awesome.
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Messing up simply means
that you're trying, and
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you can't learn without trying, can you?
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So, don't let those mistake get you down.
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It's part of the learning process,
and REPL is awesome for
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that kind of exploration.
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I do get it, though.
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Those error messages can be intimidating.
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So let's do this,
let's take a quick break, and
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then take a look at some of the more
common errors, and how to handle them.
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