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Why is __name__ == "__main__" not working?

What does the following do in Python?

  • if __name__ == '__main__':     pass When is the if statement true and when is it false? Please explain it simply.

  • Answer:

    The if statement is true when the python code is run as a script and false when the code is imported as a module. Suppose I write a bit of code that does something useful - say some numerical computation.  As a part of this code, I have written several functions.  Let's suppose the code looks something like this: #computation.py def subfunc(data): return modified(data) def otherfunc(data): return changed(data) data = load_data(filename) final = otherfunc(subfunc(data)) write_data(outfilename, final) I've tested this code, and it works great!  I can run it from the commandline as python computation.py, it reads data in from filename, performs my computation, and writes out the results to outfilename. Later, I decide that those functions I wrote were very useful, and I'd like to use them again in another piece of code.  I know about the dangers of copying and pasting code, so I'll just use import to load them directly from this file.  Unfortunately, when I try and do something like from computation import subfunc not only will I load in the function I want to use, it will also try and execute all the other code in the module.  So it will try and load in data from filename, modify it, and write the data back out.  But in this case, I don't care about all that --- I just want the function.  So the thing to do is to go back to computation.py and modify it with an if statment: #computation.py def subfunc(data): return modified(data) def otherfunc(data): return changed(data) if __name__ == '__main__': data = load_data(filename) final = otherfunc(subfunc(data)) write_data(outfilename, final) Now the code in that if statement will run just fine if it's the main script - i.e., if I run it as a stand-alone script.  But if it gets run as a part of an import, then it won't be the main script and that loading/writing part won't be done.

Brett Olsen at Quora Visit the source

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A python module is a file containing Python definitions and expressions. The module name is the name of the file excluding the .py extension. Inside the module __name__ is a global variable which has the name of the module as a string. E.g. if you have a python file sum.py: # sum.py import sys def sum(n): '''Returns sum of first n numbers''' return n * (n + 1) / 2 if __name__ == '__main__': print '__name__ is __main__' print sum(int(sys.argv[1])) You can use the module in other python files. If you run the following on your python prompt, you'll get the name of the module. >>> import sum >>> sum.__name__ 'sum' >>> sum.sum(5) 15 In the import case, since __name__ is 'sum' the condition of the if statement fails and the code below it does not run. Python modules can also be run as independent scripts like: $ python sum.py 5 __name__ is __main__ 15 Here, python sets the variable __name__ to '__main__' , which is utilized to make the module to be used as an independent script as well. In this case the if statement would pass.

Akshit Khurana

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