What decides which python class' main() is run

Question

I'm trying to make a tool that displays parameterized data on a dedicated monitor.

Following what I've learned in java, I name the method containing the "calling" code main(). I then call this main() at the end of the file. The problem is, for some reason when I run python file1 in the command prompt, it runs the main from file2. File1 imports file 2, but since avoided the namespace collision over main() by not writing from file2 import*, I'm not sure it's running the other file's main().

I'm running windows 7 Ultimate, python 3.4

@Engineero just the statement main() at the last line, without a class specifier, I assume that would default to the local class in file 1 — AllTradesJack, Jun 24 '14 at 00:04
possible duplicate of [What does \`if \_\_name\_\_ == "\_\_main\_\_":\` do?](http://stackoverflow.com/questions/419163/what-does-if-name-main-do) — otus, Jun 24 '14 at 09:55
@joshsvoss, are you also calling main in file 2? In that case the above link will help you. — otus, Jun 24 '14 at 09:57
Everything written in the global scope of the module is executed when the code is loaded. In your case, when you import file2, the function is called. Do not call anything directly in the module scope unless you know what you're doing; for executing a main function you can check the special variable `__name__` which is set to `"__main__"` if the module is the main module, and the module's name otherwise. See the question linked by otus for more information. — l4mpi, Jun 24 '14 at 09:58

score 2 · Answer 1 · edited May 23 '17 at 12:10

2

Look into using the __name__ variable to specify which main() you want to run.

For an example, check out: What does if __name__ == "__main__": do?

edited May 23 '17 at 12:10

Community

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answered Jun 23 '14 at 23:58

gr3co

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pepr · Accepted Answer · 2014-06-24T12:21:28.543

The main is just a Python variable that points to the function object. If you assign the main a different value, you loose the reference to the original function. If you define the main after the import, then your own main will be used.

Update: If both file1.py and file2.py contain the definition of the main() function and if both call it in the file -- say as the last line in its own file, and if the file1.py does import file2--but does not from file2 import * after the main() definition in the file1.py--then both main() functions will be called. The file2.main() is called during the import file2 (the first import in the application only), and the file1.main() is going to be called when the main() call in file1.py is found.

Whenever xxx.py is launched as a script, it is processed in the order of the source text. Processing of a definition means compilation of the definition, when a command is found during the first read, it is executed (actually after compilation to xxx.pyc first).

The only way to avoid execution of a code in your file is to jump over the code fragment using if with suitable condition. This is why the pattern

if __name__ == '__main__':
    main()

is used in the sources. (Which actually is the execution of the code -- only the branch of the if is skipped.) Read it... If the file is launched as a script, then the __name__ variable takes the string value '__main__', the condition holds and the main() from this file is called.

If the same pattern is used in the file2.py, and the file is used as a module (that is import file2), then the __name__ variable contains the name of the imported module (here file2), and the condition does not hold. Because of that, the main() from inside file2.py is not called.

The pattern is often used even in the files that are expected to be used as modules only. The code is usually used for testing the basic functionality of the module. Think about the situation when the author of the module makes some changes. Then he/she launches it as a script (which is not usually doney), and the body starts testing of the module and reports the result -- say using unittest module or whatever kind of testing. So, the file2.main() would be a good place to activate unit testing, for example.

Try to add the following to the end of your file1.py:

print(__name__)
print(file2.__name__)

What decides which python class' main() is run

2 Answers2