You should put this in all your Python scripts | if __name__ == '__main__': ...

Key Takeaways

hello and welcome I'm James Murphy from M coding and today we're going to be talking about the deaf main if name main idiom in Python and why you should be using it in all of your scripts first off not every file is a script by a script I mean a file that you intend to run not a library so something like this that just defines a function that you're supposed to import in another file is not going to benefit from this idiom most languages like C plus plus or Java have a kind of boilerplate setup that you need to write even to write the most basic function Java is notorious for having its public static void mainstream args as one of the first things that a programmer sees before learning to use it but in Python it's seemingly a lot simpler all you need to do is say print hello world but what I hope to convince you of by the end of this video is that you should really prefer this deaf main if name main idiom over just writing things out in a line okay really quickly if you're not familiar what is this name variable if you run the script from your terminal or elsewhere you'll see that the name variable turns out to be this double underscore main double underscore but if you import the file you see something different in this case we see the double underscore name is if name main package.import me to print name the difference in Behavior allows you to check whether the file is being run as a script or not if you see done domain then it's being run as a script and if it's anything else then it's being imported you could just stop there and put all of the statements that you want to execute when this is run as a script into this if block but I say you should go one step further and Define it as a main function okay so you can check whether or not your code is being run as a script or being imported but why why would you want to do this is it just a way to flex on programmers that don't know the idiom or is there a reason for actually doing this well yes of course there are reasons for doing it let's go over them the first and most important reason actually has nothing to do with the language itself and everything to do with how people use it yes you can use if name main to check whether or not your code is being run versus imported but this isn't what it signals to the person reading your code if name main is used to signal that this file is a script and you can run it if I look at a file and I don't see if name main I assume that I cannot or should not try to run it it makes me think it's supposed to be a library or that it's only meant to be imported nothing about the Python language actually enforces this but compare it to using an underscore as a variable name if you see an underscore in someone's code they're basically saying I'm not going to use this variable Nothing Stops them from using it but if they do use it they're kind of breaking an unspoken rule in the python community underscore means I'm not going to use this variable and if they main means this is a script don't keep your reader guessing if this is supposed to be an entry point to your program just make it explicit many editors like pycharm use this as a signal you can see the file on the left has this green arrow which says run hello but there's no green arrow on the right that's because pycharm knows the signal if they main means this is something that I want to run no if name main means this is probably a library you're not meant to just run it but why should you extract main out into a function instead of just leaving the contents in the if block here's a script that just defines some function and then uses it to compute something even if I put this code inside a nameguard everything that I do here is still in the global scope in particular this variable I is a global variable even if I didn't really mean it to be that suppose I Define a function to compute some value and then later on I use it here I'm just trying to add up some evaluations of this function based off of this list but I have accidentally made a typo I use the variable I here but the name of the loop variable was n when I run the code instead of getting an error for my typo I silently get the wrong value this is one of the worst kind of errors that you can have and this is all just within the same file imagine if this happened in a different file if in another file or at the console I do an import Star of the bad script I now have a global variable I that I didn't know about so if it was this file that defined the compute Val function this file now has a global variable I and it's even harder than in the last example to find this typo also in the case where you didn't have an if name main at all just importing the script will actually run it that basically means that it's impossible to test your code without side effects also note that there are plenty of cases where importing your code happens automatically this script explicitly Imports the useful class from the bad script and then writes it to a pickle file it's clear here that we're importing the bad script because we explicitly wrote it but in this script there's no mention of the bad script all we're doing is unpickling some object we don't even know where it came from and yet when we run this code we see that we did in fact import the bad script and run all of the code that was in its body this is because pickle.load automatically imported the bad script in order to get the definition of the useful class in the pickle case it was just a little bit annoying to have that extra stuff printed out but in this case we're doing some multi-processing the goal is to compute the values of this useful function over some list of inputs multi-processing pool is going to start up a bunch of different python processes that all automatically import this file but without an if name main you can see how this starts to get into trouble just importing the file starts up new processes which then import the file and so on so you pretty much can't use multi-processing in this case look what happens and I can't even kill it with Ctrl C I have to kill the terminal putting this code in an ifnay main prevents the rampant process spawning but again without putting this in a separate function this is now going to give me a global variable named p all you have to do to avoid this headache is just use the idiom it's not that much typing and in fact you could probably just set up your Editor to have it there automatically when you make a new file or just have a template to paste it when you type Main one final less used reason for making a main function is that this allows you to have an entry point to your program that you can use from other scripts so I don't have to run this from the command line I could import this from another file and then call the main function as if I was running it from the command line without starting a new process in this case you can have your main taken whatever arguments as an optional parameter all right that's all I've got on the deaf main if name main idiom let me know if you found any other reasons to use it also let me know if you're a holdout what your reasons are for not using it as usual thanks to my patrons and to my donors for supporting me if you like the video don't forget to subscribe and hit the like button see you next time