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Let's talk about '__name__', and '__main__'.

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You're going to see this a lot in Python files.

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So often in a file, you're going to see something like, 'if

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__name__ == "__main__":',

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[no audio]

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and then 'print("Do a thing")', or maybe it runs some sort of

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function. Now, this is actually pretty important when it

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comes to running a Python script.

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And this is again, fairly common.

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You're going to see this quite a bit out there when you start

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looking at other people's Python code.

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Now, the idea behind this is to determine if the file is

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being used as, like an import for a module or a package,

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or if it's being used by a regular Python script,

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so if it's basically just supposed to run as is.

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So is that an import, or is it running like a program?

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So I'm going to create a new file, new folder in here, and,

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nope, not in there.

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And this is going to be called 'Name and Main'. Clear that. 'Name and

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Main', create a new file.

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And this one's just going to be called 'first.py'.

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And I just put it in a new folder just to sort of separate it

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from the other files

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so we don't get a little confused about which files do what.

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Now, how Python works, is you can sort of see this invisible

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line that comes up and down in my editor here.

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Any code that is the farthest left, Python is going to,

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it's going to try to run that,

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it's going to at least try to analyze it.

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So if we said variable 'name = "Kalob", and the 'course

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= "Python for Everybody", then these are going to

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be evaluated right away.

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But if we said 'def', some function name, 'print("a thing here")',

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the reason this automatically doesn't get run is because

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this code here is not touching this side.

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So Python is going to go through here, and say, "On line 2

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there's a variable, on line 3 there's a variable, and on

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line 5 there's a function."

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But this code here is not touching the wall.

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It's actually indented inwards.

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And because of that, Python says, "Okay, I understand that

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there is a function here, but I'm not actually going to run

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this". And in every Python script, there's always this variable

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called '__name__'.

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So let's go ahead and get rid of some of this stuff.

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And let's just print name, 'print(__name__)', and let's run

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this in our terminal.

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So 'cd' into the folder called 'Name\ and\ Main/', 'ls -la' or 'dir'

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if you're on Windows.

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And let's do 'python first.py', and it says, this '__name__' is '__main__'.

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So now we know, 'if __name__ == "__main__"', that this is a program

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calling the file, and we can say 'print()' in here,

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"This is a program running the file".

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And let's go ahead and run that code in our terminal just once more.

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Sure enough, "This is a program running the file".

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[no audio]

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But just for funsies, let's also add an 'else' statement in here.

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'else print("This is an import")'.

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And when we run this, we're going to see that nothing actually changes here.

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But if we go ahead and create another file, let's call it

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'second.py', and it's right beside my 'first.py', I can say

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'from first import', that's this file here, I can import something in here.

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So let's create a function called 'greeting()', and it's just

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going to say, 'print("Hello from first.py")'.

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Let's get rid of this 'print' statement, that's not useful anymore.

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And so in 'second.py', we are going to import that 'greeting()'

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from the first file here,

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'first.py', and because '__name__' is not going to be "__main__", because

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it's actually just an import,

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this should run for us,

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and it should say, "This is an import".

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So let's go ahead and do this, 'from first import greeting',

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and then execute the function,

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and let's run the script with, 'python second.py'.

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It says, "This is an import", because, hey, look at that,

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we imported the file before we did anything, and then we

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ran the script.

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And because of that, it says "This is an import", and then

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ran the function that we're looking for.

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[no audio]

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And again, if we run 'python first.py',

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says, "This is a program running the file", and if we run 'python

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second.py', it says, "This is an import".

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Now, you actually aren't going to see this 'else' statement too

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often. It's pretty rare that you see this kind of 'else' statement.

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Usually you just see something like this.

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And in fact, you won't even usually see a 'print' statement.

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But what you will see is if Python is calling this file as

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a program, you will often see a function being called in

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here. So this function is already defined, and what this

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is saying is just automatically run that function.

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So let's go ahead and run 'first.py'. 'python first.py'.

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And it says, "Hello from first.py".

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And that's because it is now executing this automatically.

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And in 'second', we don't actually have anything in here.

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So even if you run 'second.py' as a program, it doesn't really care.

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We're not telling you to do anything extra.

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And if you run it as an import, it's not going to do anything

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anyways, because it's just an import.

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So if you ever need to determine the difference between when

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a file is being run as a program versus a file being imported

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as a module or a package, this is the way you would want to do that.

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Now you might be thinking, "Why on Earth would I ever need

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that?" Well, let's say you have a little mini search engine

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or a little web scraper that you've built, and you have a

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function in here that scrapes a page, 'scrape_a_page', and it

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just goes ahead and gets all the images from a particular

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website. So it's going to work some magic in here,

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[no audio]

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and it's going to 'return "something", '#magic logic in here'.

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Now, that's fine.

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'scrape_a_page' might automatically want to be run as soon

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as you call 'python first.py'.

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Now, if you wanted to reuse this, you could copy this whole

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file into another project that you're working on,

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and you could simply import.

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You could say 'from first import scrape_a_page'.

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And because this isn't in here, it's not automatically going

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to scrape the page. So

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now you can actually use it as a regular import, but you can

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also use it as a regular program file.

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Now, what I would like you to do is test this out, create

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'first.py', 'second.py', and in your 'second.py' import from your

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'first.py', and have this '__name__ == "__main__"', and then

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run a function in here.

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Now I'm going to undo a bunch of this because that's not actually

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going to do anything for us.

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Go ahead, give this a shot just like I did in this video.

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You may or may not use this all the time, depending on sort

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of which industry you're going to go into with Python.

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But it is still good to know because you are going to see this

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regardless in Python code.

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[no audio]