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Learn a Python best practice for working with larger projects that include more than one file.
Terms
File - A file that contains code and ends with .py
Script - The file you execute from the command line
Module - The file(s) that are imported into other modules or scripts
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[MUSIC]
0:00
Hi there, my name is Megan and
I'm a teacher here at Treehouse.
0:09
In this workshop, we're going
to be talking about dunder main.
0:13
Dunder main is going to become your best
friend when you start working with more
0:17
than one Python file.
0:21
The best way to understand
what dunder main is and
0:23
why you need it is to jump into the code.
0:26
So let's go.
0:29
I'm going to create two files here.
0:31
Our main file I'm going to call
app.py to follow typical conventions.
0:33
And the other one I'm just gonna call
dm for dunder main, .py, perfect.
0:39
Now, typically, when you start
needing more than one file for
0:46
your Python projects,
your app.py file will control
0:50
the application while other
files hold necessary code.
0:53
It helps keep things less cluttered and
also makes it a lot easier to find things.
0:57
app.py is called a script,
1:03
because it is the file we intend
to run from the command line.
1:05
The dm.py file is called a module,
because we will be using the code
1:10
written in this file to import
it into our main app.py file.
1:16
Modules are meant to be imported
into other modules or scripts.
1:22
Inside both files,
I'm going to add a simple print statement.
1:29
So let's do print, and I'm just gonna do
hello from dm.py, don't forget to save.
1:33
And pretty much the same thing in here,
1:41
print("hello from app.py"), save.
1:45
Now, if I run both individually,
down here, 3, I'll do dm first and
1:49
I get hello from dm,
because that's what we have in that file.
1:56
And then if I run app.py,
I get hello from app.py.
2:02
So it works exactly as we expect.
2:08
But these files are meant to be
working together, not apart.
2:10
So let's import dm into app.py.
2:14
So up at the top, import,
dm, I'm gonna hit Save.
2:19
And now I'm gonna do the same thing again.
2:26
I'm gonna run dm.py first,
2:28
and I still get hello from dm.py.
2:32
Now let's see what happens
when I run app.py.
2:36
Now you can see I am
getting hello from dm.py.
2:46
So it's running this file first and
2:50
then it's running what is
here inside of app.py.
2:53
So I'm getting two print statements
printed out to the console,
2:57
even though I'm only running one file.
3:01
This may not seem like a big problem.
3:04
But imagine having a huge website or
application with like 100 files, and
3:06
all of a sudden there's all
these print messages and
3:11
all sorts of things just
taking over your console.
3:14
Now, this is where dunder main comes in.
3:18
It looks like this, and
we're gonna add it to our dm.py.
3:20
I'm gonna space it down here a little bit,
3:22
and it's gonna go if dunder name,
3:27
which just stands for double underscores,
3:31
dunder name equals dunder main.
3:37
And then let's Tab this so
that it's inside of our conditional.
3:41
We don't really need that
extra line at the top.
3:44
Okay, so now that I have that saved,
let me run our two files again.
3:48
Now, I'm just gonna make this easy,
I'm gonna clear the console just so
3:52
we can tell pretty easily.
3:57
So I'm just gonna use the up
arrow to run dm.py again.
3:59
And you can see, when I run dm.py,
I still get hello from dm.py.
4:02
But now let's run app.py.
4:07
I want you to pause and think,
4:09
what do you think is going to
happen when I run this file?
4:11
Just a quick thought before
I hit the Enter key.
4:14
All right, are you ready to see?
4:19
Okay, now we are just
getting hello from app.py.
4:22
So we are still importing our dm.py file,
but
4:27
we're no longer running
this print statement.
4:30
It is only running the print
statement here inside of app.py.
4:35
In the next video,
4:41
we're going to dive into dunder main
some more to see why this happens.
4:42
See you there.
4:46
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