Why is Python running my module when I import it, and how do I stop it?

I have a Python program I'm building that can be run in either of 2 ways: the first is to call "python main.py" which prompts the user for input in a friendly manner and then runs the user input through the program. The other way is to call "python batch.py -file-" which will pass over all the friendly input gathering and run an entire file's worth of input through the program in a single go.

The problem is that when I run "batch.py" it imports some variables/methods/etc from "main.py", and when it runs this code:

import main

at the first line of the program, it immediately errors because it tries to run the code in "main.py".

How can I stop Python from running the code contained in the "main" module which I'm importing?

Because this is just how Python works - keywords such as class and def are not declarations. Instead, they are real live statements which are executed. If they were not executed your module would be empty.

The idiomatic approach is:

# stuff to run always here such as class/def
def main():
    pass

if __name__ == "__main__":
   # stuff only to run when not called via 'import' here
   main()

See What is if __name__ == "__main__" for?

It does require source control over the module being imported, however.

Due to the way Python works, it is necessary for it to run your modules when it imports them.

To prevent code in the module from being executed when imported, but only when run directly, you can guard it with this if:

if __name__ == "__main__":
    # this won't be run when imported

You may want to put this code in a main() method, so that you can either execute the file directly, or import the module and call the main(). For example, assume this is in the file foo.py.

def main():
    print "Hello World"

if __name__ == "__main__":
    main()

This program can be run either by going python foo.py, or from another Python script:

import foo

...

foo.main()

Use the if __name__ == '__main__' idiom -- __name__ is a special variable whose value is '__main__' if the module is being run as a script, and the module name if it's imported. So you'd do something like

# imports
# class/function definitions
if __name__ == '__main__':
    # code here will only run when you invoke 'python main.py'

Unfortunately, you don't. That is part of how the import syntax works and it is important that it does so -- remember def is actually something executed, if Python did not execute the import, you'd be, well, stuck without functions.

Since you probably have access to the file, though, you might be able to look and see what causes the error. It might be possible to modify your environment to prevent the error from happening.