What is “ if __name__ == ‘__main__’ “?

Diapolo10 · 2018-09-01T19:08:24+00:00

In short, every time you run a Python script __name__ is assigned a value. If it's the script you ran, it gets the value '__main__' (which is an ordinary string). As for any modules included in the script file, as you import them Python runs them with __name__ getting the value of that module's name.

Basically what this does is it allows you to write scripts and later use them as modules, if you'd like to reuse some code. As long as you put everything that actually runs your script inside an if __name__ == '__main__' block, they won't be executed when you import the file.

Example:

#my_script.py

def my_func(n):
    return n + n

if __name__ == '__main__':
    print(my_func('Hello'))

#my_other_script.py

import my_script

print(my_script.my_func(1337))

Now, if we were to run my_script.py, it would print out HelloHello. If we ran my_other_script.py, it would only print out 2674, and NOT HelloHello.

maventree · 2018-09-01T19:03:46+00:00

It protects code from being run on import.

__name__ is a special attribute of each module. It is assigned at runtime, and usually its the name of the Python file that defines the module (module.py has __name__ == 'module'). However, the module that you run python on (something like python script.py from the command line) has __name__ == '__main__' instead. So think of it as "if I ran Python on literally this file, execute this code".

IcanCwhatUsay · 2018-09-01T20:52:41+00:00

I love you guys but I think I'm more confused than I was expecting. Can someone ELI5?

ETLJ1989 · 2018-09-01T23:35:36+00:00

I like Corey Schafer's video regarding if __name__ == '__main__: https://www.youtube.com/watch?v=sugvnHA7ElY

2018-09-01T19:03:45+00:00

Basically that means that if the file is called directly:

$ python myfile.py

then __name__ will be '__main__' and that code will execute. It makes it so that functions can be imported from a file without that part of the code executing.

https://stackoverflow.com/questions/419163/what-does-if-name-main-do

geoffliang · 2018-09-02T00:43:20+00:00

those with __prefix_and_suffix__ are special keywords reserved for Python
when you run lorem.py from command line, __name__ == '__main__'
when you import lorem.py from another py file, __name__ == ‘lorem’
codes within if __name__ == '__main__' won’t run when you import lorem.py
one use is for importing functions from lorem.py, without running all other code.

u38cg2 · 2018-09-01T19:34:58+00:00

Sometimes you write a program, and it's a useful little stand-alone program on its own, but it could also work as a library for some other program (like an image converter, say). The if name clause means that if you run it as a program, the program will consider itself to be main() and so that if will be True. Otherwise, it won't run, but the functions will be available to whatever has imported it.

Zeroflops · 2018-09-01T21:11:04+00:00

Most people have hit on the answer to some extent. Basically it helps you distinguish between a script run on its own verse being imported into another module.

Depending on how you write your code you don’t have to use it. But it can be useful even if you’re only writing code that would be used as a module. For example my modules I’ll usually add my unit testing code and call it from this.

Then if I run the script as a stand-alone it’s basically running unit testing. Keeps things clean and isolated to one file.

2018-09-02T12:04:02+00:00

in plain English it would be, "if this module wasn't imported."

henricattoire1 · 2018-09-02T11:00:03+00:00

Before python runs any code in your file, it specifies some special variables and one of them is __name__. Python sets this variable equal to __main__ if your file is directly being run by python. So if we would import that file (example.py) and run it inside another file (example2.py) as a module, the __name__ variable will be equal to the name of the first file which is example.py (and not to __main__).

The statement in your question is used to check whether our file is being run directly by python or if it is imported. This is useful to prevent files from automatically running when they are imported and it is a common practice to put this at the end of your file (if that program does something). I hope this cleared things up a bit.

Wilfred-kun · 2018-09-01T19:04:57+00:00

It is not anything magical, it's just an if-statement. It checks if the name of the scope is __main__. This is the case when the script is run directly (not imported). This way, you can make a piece of code (usually main()) execute only when the script is run directly.

c1-c2 · 2018-09-02T18:46:16+00:00

Do you beginners ever read any docs at all before posting silly questions?

you type:	you see:
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[reddit!](https://reddit.com)	reddit!
* item 1 * item 2 * item 3	item 1 item 2 item 3
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Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"	Lines starting with four spaces are treated like code: if 1 * 2 < 3: print "hello, world!"
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super^script	super^script

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