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Previously, you learned that to execute a Python program in Visual Studio Code, you can either use the 'Run'

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button or type in the command manually in here, such as python3.9 or python3.10

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or python3.8, depending on what version of Python you have.
If you are on Windows...

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So that was for Mac and Linux. On Windows, that'd be python3.9 and then that would be followed by

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the name of the file.

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In this case, 

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basics.py..

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So that file that contains the code.

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But what happens if you delete the name of the file and executes only the first part of the command?

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This would happen.

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So when you don't apply a file to the python command, python will open in an on-demand mode.

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This is also known as the python interactive shell or just the python shell for short or the python

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console.

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So different names for this on-demand Python interpreter.

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This interactive shell expects python codes directly.

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So all you have to do is write the code 24 times seven, for example, and press enter and you get the

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output instantly.

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And so the purpose of this python shell is to give programmers a quick interpreter useful for testing

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things out and playing around with the code.

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For example, while we write this program that calculates the amount of hours in a week, we may not

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be sure about certain features what we need to use.

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For example, we don't know whether to use an asterisk or a plus for the multiplication operation.

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So in that case, we would open the python shell just as I did in here and try things out.

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So is the asterisk a multiplication operator or is the plus what we are looking for?

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And you see that this doesn't make much sense.

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So it looks like we have an addition operator here and a multiplication operator here.

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So then we go ahead and write that in our Python file in our program.

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Of course, this was a very 'dumb' example because we all know that the plus is for additional operations

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and so on.

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But in more complex programs, you have to try things out to see what works and what doesn't work here

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in the interactive shell in a quick fashion and then

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write that code in the python file.

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So that is the advantage of the python shell quick programming.

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The disadvantage is that the code you write in the python shell cannot be saved.

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So if you close Visual Studio Code now and you open Visual Studio Code again, this code here

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has disappeared, the shell will close, this session will close, and you will not see the code anymore.

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In contrast, you can save the code in dot py files because they are files saved in disks.

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Therefore, the conclusion here is that the python shell is for testing things out and .py files are

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for writing complete programs.

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Now I want to give you a pro tip before I close this video.

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I don't know if my computer has a calculator app or not, and that is not because I never needed to

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use a calculator, but because I never look to see if my computer has a calculator because I use Python

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as a calculator. Specifically,

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what I use actually is that interactive shell and I don't open the interactive shell through

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Visual Studio Code, but through the computer terminal on Windows that is called cmd.

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So you should go to search.

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Look for cmd or the command prompt, open that.

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Write py-3.9, or 3.10, whatever interpreter you are using,

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execute.

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And then you can start using Python as a calculator.
On Mac you would use

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of course, python3.9, the command and you'd search for terminal amoung your programs,

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not cmd.

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That is the case for Linux users as well.

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So then here you can use Python, just as we did here in Visual Studio Code.

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And actually, it would make more sense to use the interactive shell for the program that we built instead

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of writing the program in here.

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So we wanted to know how many hours are in a week and you can just use the shell for that because you're

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not building an application that is going to be used multiple times.

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So this is just a computer program that calculates something.

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Of course, that's not a clear distinction between programs and applications, but if there is one,

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it is that applications are supposed to be used many times by users and programs can be throw away scripts.

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So this is more like a program.

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And throughout the course, we will build 10 programs which look more like applications.

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And that's about the Python interactive shell.

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One warning you should be aware of is that if you run your program now while the interactive shell is

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open, see what's going to happen.

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I press the 'Run' button.

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We get the output's, so this comes from this program, but be aware that if you open an interactive

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shell in this terminal now,

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and then you run the program again, you're going to get an error because what Visual Studio Code is

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trying to do is that it is sending the Windows commands,

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so the Python interpreter path and the file path of basics.py.

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So these are commands supposed to to go to the Windows command line.

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But this one here is the Python interactive shell and this expects Python code.

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But this is not Python code.

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That's why we get this python error because python is confused with this code.

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So it's a syntax error.

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Therefore, if you see this error, what you should do is you should exit

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the python shell with this command, it's a python function that exits the interactive shell or a program

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in case you use it in a dot py file, execute that and you are back to the terminal and here you can

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executes that program again.

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Now, if you look here, you'll see that we have 
two terminals actually. This one is the one we were

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using, but then when you execute that, a new terminal is created.

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So this one was created and then here in this terminal,

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we opened the python shell with that.

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And then when you execute the program, this terminal, it will be used again

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by Visual Studio Code, therefore, a trick you could do is that... 
Let me delete these two terminals

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so I have nothing here.

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What you could do is to use one terminal to run the dot py files.

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So this one here, you run it, you run it, and then create a new terminal by using this, so split terminal and you're

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going to get two terminals.

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In one, you keep a python interactive shell open, and the other you use it to see the output of your dot py files.

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So you execute that.

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You get the output here and this one here, you use it for your own testing.

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Again, if you click that button, the output is going to always display in this terminal.

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And with that, thanks a lot for following this video and I'll see you in the next one.