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The Python Shell6:06 with Craig Dennis
Learn how to use the Python interpreter interactively
We just saw how to pass a script to the Python interpreter. 0:00 Another way to use the interpreter is in a more exploratory interactive way. 0:03 You can actually open up a prompt that allows you to type Python code 0:07 line by line. 0:10 This is super handy when you just wanna see what some code does, and 0:11 not actually go and create a whole script. 0:14 This type of interactive programming prompt is pretty popular in other 0:16 languages. 0:20 Generically this this type of exploratory prompt is referred to as a REPL or 0:21 R-E-P-L. 0:26 which stands for Read, Evaluate, Print, Loop, which is basically what its role is. 0:28 It reads the line, it evaluates it, it prints the result and 0:32 then it loops back so you can add another line of code. 0:36 Python's REPL is often referred to as the Python shell. 0:39 It is a wonderful place to hang out as you are learning, 0:42 so I definitely want you to be familiar with it. 0:45 Come on, let's go explore. 0:47 So, to open up the REPL we simply type Python. 0:49 And we'll get some information about the version of Python that we're running. 0:55 So we're running 3.6.4 on Linux. 0:59 Remember, your workspace is running a Linux OS, or operating system. 1:03 These three greater than signs, or chevrons, as they're sometimes called, 1:07 are communicating that this is the place where we can write some code. 1:11 So, let's do it. 1:15 Since we know we have a working program up here, let's just write that out ourselves. 1:17 So, we want to make sure that we type it exactly like it is above. 1:21 Make sure that you keep everything lowercase, case matters in Python. 1:25 So, this prinT is different than the lowercase print. 1:30 So, that's the name of the function that we want to call, and 1:34 you call a function using parenthesis. 1:38 And now we wanna pass in a string. 1:41 And we can create a string using quotation marks. 1:43 We'll do that, open that up. 1:46 And then our characters, which here happens to be, Hello, World. 1:47 And then we close our string with another quotation mark. 1:53 And then, finally, we close our function call with a closing paren. 1:58 So let's go ahead and run that. 2:02 Awesome, Hello, World. 2:05 Now, a great thing about the shell is that it keeps it's history. 2:06 If you press the up arrow, you can get the last line that you just typed back. 2:10 This is handy if you had made a typo or 2:13 you wanna write a line that is very similar. 2:15 Like for instant, let's just say hello to you. 2:17 So I'm gonna put my name in here. 2:20 Hello, Craig. 2:21 Awesome, and Python shell is also a pretty good calculator. 2:23 You can use it to do math-like things, things like 1 + 2. 2:26 And so you'll see here that the result printed out 3. 2:30 Note that we didn't actually call the print function. 2:34 Now this is a good example of the REPL in action. 2:38 What happened was it read the line, 1 + 2, it evaluated it, what's 1 plus 2? 2:40 And then it printed the result, 3,and it looped back to the prompt. 2:46 It showed the result so that we could see it. 2:50 We'll get to some more math in the course in just a bit. 2:52 Now another thing that is wonderful about the Python prompt 2:55 is that you can help when you need it. 2:58 So, for instance, if we wanted to know more about the print function, 3:01 we could just call the help function. 3:04 So we say help and we pass in the function that we're interested in, 3:06 we're interested in print. 3:10 Let's see what happens. 3:11 So this kicks open the documentation for the print function. 3:13 Now, this is gonna have some terminology in here that we haven't yet 3:16 covered, so don't let that overwhelm you. 3:19 Now believe it or not, if you stick with it and 3:21 immerse yourself, this will all make sense. 3:23 We'll get to all of this, just not right now. 3:26 So here's the description, it prints the values to a stream, or 3:29 sys.stdout by default. 3:32 Standard out is another way of saying the place that you ran the program from, 3:35 the default output. 3:40 So basically, what this is saying is that we can print to other places, too. 3:43 This value here is the hello world string that we passed in. 3:47 Then you'll notice that there's a comma and then there's these ellipses, 3:51 there's these three ellipses here, right? 3:54 So, this means that we can actually pass multiple values to print, and 3:56 we'll do that here in a bit. 4:00 We'll pass multiple values. 4:01 Now, if you look down at the bottom here you'll see that that says END and that's 4:03 because we're inside of what is known as a pager, when help opens it does that. 4:06 Now, it just so happens that all the help fits on one page but let's go ahead and 4:11 let's make that not happen. 4:14 So this is something you can do too, you can make the console bigger or smaller, so 4:15 I'm going to make it smaller. 4:18 And you'll see what happens is, eventually, 4:19 there's this like blinking colon. 4:22 And I wanted to show you this just in case you opened up help to something else. 4:24 You can press the up and down keys and move around. 4:27 And space bar actually moves a page at a time. 4:29 So, to get out of a pager, what you can do is press Q. 4:32 And now normally what would happen when you popped out of here is you would pop 4:39 back into your shell. 4:42 But it looks like I had a little bit of a workspace problem. 4:43 Which I'm glad happened, so I can show you what to do when this happens. 4:45 So the console, 4:49 you can restart your console always by clicking either this X here. 4:50 This will close the console. 4:54 And I can up here and say View > Show Console. 4:55 It should pop back open, there we go. 4:59 And so if we are inside of a shell, 5:02 You might wanna know how to get out of here too, right? 5:04 So, there is a handy function called exit, and that will pop you out. 5:07 But even better, and I know cuz we programmers are lazy, 5:12 you can also press Ctrl + D to drop out of there. 5:16 There we go. 5:20 Awesome, so now you have a place to go and explore when you need to. 5:21 Also, you can't break anything in there. 5:25 So, feel free to do whatever. 5:27 And like you just saw, you can always exit out. 5:28 A common thing that happens to people just beginning to learn to code 5:31 is that they're afraid to make mistakes, so they freeze up. 5:35 Please don't be afraid of making mistakes, that will bite you in the future. 5:38 Try to change your point of view to this one, making mistakes is awesome. 5:41 Messing up simply means that you're trying, and 5:45 you can't learn without trying, can you? 5:48 So, don't let those mistake get you down. 5:50 It's part of the learning process, and REPL is awesome for 5:51 that kind of exploration. 5:54 I do get it, though. 5:56 Those error messages can be intimidating. 5:57 So let's do this, let's take a quick break, and 6:00 then take a look at some of the more common errors, and how to handle them. 6:02
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