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[MUSIC]

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Hi there, my name is Megan and
I'm a teacher here at Treehouse.

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In this workshop, we're going
to be talking about dunder main.

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Dunder main is going to become your best
friend when you start working with more

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than one Python file.

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The best way to understand
what dunder main is and

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why you need it is to jump into the code.

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So let's go.

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I'm going to create two files here.

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Our main file I'm going to call
app.py to follow typical conventions.

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And the other one I'm just gonna call
dm for dunder main, .py, perfect.

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Now, typically, when you start
needing more than one file for

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your Python projects,
your app.py file will control

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the application while other
files hold necessary code.

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It helps keep things less cluttered and
also makes it a lot easier to find things.

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app.py is called a script,

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because it is the file we intend
to run from the command line.

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The dm.py file is called a module,
because we will be using the code

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written in this file to import
it into our main app.py file.

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Modules are meant to be imported
into other modules or scripts.

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Inside both files,
I'm going to add a simple print statement.

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So let's do print, and I'm just gonna do
hello from dm.py, don't forget to save.

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And pretty much the same thing in here,

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print("hello from app.py"), save.

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Now, if I run both individually,
down here, 3, I'll do dm first and

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I get hello from dm,
because that's what we have in that file.

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And then if I run app.py,
I get hello from app.py.

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So it works exactly as we expect.

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But these files are meant to be
working together, not apart.

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So let's import dm into app.py.

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So up at the top, import,
dm, I'm gonna hit Save.

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And now I'm gonna do the same thing again.

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I'm gonna run dm.py first,

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and I still get hello from dm.py.

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Now let's see what happens
when I run app.py.

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Now you can see I am
getting hello from dm.py.

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So it's running this file first and

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then it's running what is
here inside of app.py.

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So I'm getting two print statements
printed out to the console,

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even though I'm only running one file.

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This may not seem like a big problem.

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But imagine having a huge website or
application with like 100 files, and

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all of a sudden there's all
these print messages and

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all sorts of things just
taking over your console.

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Now, this is where dunder main comes in.

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It looks like this, and
we're gonna add it to our dm.py.

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I'm gonna space it down here a little bit,

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and it's gonna go if dunder name,

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which just stands for double underscores,

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dunder name equals dunder main.

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And then let's Tab this so
that it's inside of our conditional.

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We don't really need that
extra line at the top.

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Okay, so now that I have that saved,
let me run our two files again.

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Now, I'm just gonna make this easy,
I'm gonna clear the console just so

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we can tell pretty easily.

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So I'm just gonna use the up
arrow to run dm.py again.

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And you can see, when I run dm.py,
I still get hello from dm.py.

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But now let's run app.py.

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I want you to pause and think,

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what do you think is going to
happen when I run this file?

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Just a quick thought before
I hit the Enter key.

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All right, are you ready to see?

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Okay, now we are just
getting hello from app.py.

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So we are still importing our dm.py file,
but

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we're no longer running
this print statement.

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It is only running the print
statement here inside of app.py.

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In the next video,

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we're going to dive into dunder main
some more to see why this happens.

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See you there.