Python Jumpstart by Building 10 Apps Transcripts
Chapter: App 4: Journal app and file I/O
Lecture: Using __name__ to selectively execute code
0:00 Finally let's briefly turn our attention to this line 51 here, main.
0:05 So I told you calling main like this is not ideal,
0:08 but I've been pushing off talking about why that is
0:12 and it really has to do with when your code is
0:14 used as an external module to other code, right,
0:19 either code you write or if you are writing a package,
0:22 people who consume your package and let's see what the problem is.
0:25 Let me just create a program 2,
0:33 now over here in this program 2 suppose we would like to do
0:37 this print header thing up here at the top.
0:39 This print header method, it's unlikely you would import that whole thing
0:44 just to get this but go with the idea.
0:46 Imagine I want to use this print header method up here
0:48 so I would like to do something like let's just say import program great,
0:54 and I could say program.print header.
0:58 Now let's make this super obvious what is happening
1:00 in terms of the program flow, so I'll do a print,
1:02 and say about to import program,
1:08 let's call it step 1 down here we'll do a step 2 program imported,
1:16 and then maybe a step 3 printing header
1:23 and we'll add a step 4 down at the very end here, done.
1:30 Now, if you think how we want this to work
1:32 it should print about to import program,
1:36 it should perform that silently, then it should print program imported,
1:41 then it should print printing the header and we have the little header part,
1:45 remember it's going to say --- journal app--- and then it should say done.
1:50 And it should just run without the user input,
1:52 conceptually the way we are trying to run it.
1:54 However, that is not what is going to happen,
1:57 so let me run this really quick, now it says about to import program
2:00 and we see the header, ok, that is a little early,
2:02 but let's see what else is going on,
2:04 now it says what do you want to do with your journal,
2:06 and I can apparently interact with it- weird,
2:13 ok, that was pretty unexpected,
2:15 if I hit exit it exits and then it says ok now the program is imported
2:20 so all of this from here to there, that is literally line 2 of our program,
2:26 program2.py what the heck is going on?
2:29 Well, remember the way Python defines classes, functions, variables and so on
2:35 is it literally executes this file, top to bottom.
2:40 It says ok we are going to execute importing the journal,
2:43 all the stuff that that means we are going to execute defining main,
2:45 we are going to execute defining print header and so on,
2:48 so it's all looking fine until we get to here,
2:50 this actually has side effects,
2:53 this actually starts that event loop that we saw
2:55 going around and around so in order to use this,
2:58 I kind of need to not call it when we are importing
3:02 so here it worked fine, now that I commented that out,
3:05 it worked the way we expected.
3:07 So, in Python there is a convention that allows us to say
3:11 if you are trying to run this program do some extra code
3:14 but if you are trying to be a library,
3:16 just define the features the variables functions,
3:20 classes and so on during import.
3:23 It all has to do with this special implicit variable called _name_
3:29 there is several implicit values that we can get out of this module,
3:33 one is the actual full path to the file that is executing,
3:36 that can be really handy when you are trying to find something
3:39 relative to the current module, and this name,
3:42 this one actually is the convention we are going to use,
3:45 so let me just run this, remember we are running program 2,
3:49 we are going to import this, you'll see that this should be program
3:53 and this should actually be the full path to program.
3:58 Here you can see that's the __file__,
3:58 and here you can see that is __name__,
4:01 let's make this super explicit.
4:10 So I'll put __file__, __name__, then I will run it and you will see
4:13 the files that program is that could use a space, but you get the idea.
4:18 Ok, now let's go over here, let me add those spaces,
4:23 and let's go over here and change the app that we are running,
4:25 we are going to run program itself,
4:27 so when I run program, the file did not change but check out
4:30 what happened to the name, it became __main__
4:34 so the convention in Python is when you are running a script
4:38 if it's the thing the target of execution, its __name__ is set to __main__.
4:45 If on the other hand it's being imported,
4:47 it's just the name of the Python file typically without the extensions.
4:52 Ok, so how does that help us?
4:55 Well, down here in main, we can do this,
4:59 we can say if, __name__ is this main then in that case,
5:04 that means we are trying to run this program,
5:06 right now so let's kick off the main.
5:09 On the other hand, if it's being imported,
5:11 if this is program rather than main,
5:14 just don't execute our code, just let it define the functions and variables.
5:17 So if I run program, that's this file right now,
5:20 it should be main, so it should act like our original app.
5:23 Right, it lets us list entries and exits.
5:26 On the other hand, if I am trying to import it,
5:29 whereas I am running this program,
5:32 it's not going to be called __main__, it's going to be called program,
5:36 so let's run that, now notice file is of course what it is,
5:41 name is program we did not call main
5:43 so it worked exactly like we wanted,
5:45 1 about to import program, boom- done importing, printing the header,
5:50 so if we did not have this little print bit here,
5:54 it would act exactly like you would have initially
5:58 expected program 2 to run.