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What does if __name__ == “__main__”: do?

Given the following code, what does the if __name__ == "__main__": do?
# Threading example
import time, thread

def myfunction(string, sleeptime, lock, *args):
while True:

if __name__ == "__main__":
lock = thread.allocate_lock()
thread.start_new_thread(myfunction, ("Thread #: 1", 2, lock))
thread.start_new_thread(myfunction, ("Thread #: 2", 2, lock))

2 Answers

The simplest explanation for the __name__ variable (imho) is the following:

Create the following files.

import b


print "Hello World from %s!" % __name__

if __name__ == '__main__':
print "Hello World again from %s!" % __name__

Running them will get you this output:

$ python
Hello World from b!

As you can see, when a module is imported, Python sets globals()['__name__'] in this module to the module's name. Also, upon import all the code in the module is being run. As the if statement evaluates to False this part is not executed.

$ python
Hello World from __main__!
Hello World again from __main__!

As you can see, when a file is executed, Python sets globals()['__name__'] in this file to "__main__". This time, the if statement evaluates to True and is being run.
It's boilerplate code that protects users from accidentally invoking the script when they didn't intend to. Here are some common problems when the guard is omitted from a script:

If you import the guardless script in another script (e.g. import my_script_without_a_name_eq_main_guard), then the second script will trigger the first to run at import time and using the second script's command-line arguments. This is almost always a mistake.

If you have a custom class in the guardless script and save it to a pickle file, then unpickling it in another script will trigger an import of the guardless script, with the same problems outlined in the previous bullet.

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