Home > DeveloperSection > Forums > What does `if __name__ == “__main__”:` do?
sandy taylor
sandy taylor

Total Post:46

Points:324
Posted on    May-13-2015 12:26 AM

 Python Python 
Ratings:


 1 Reply(s)
 416  View(s)
Rate this:
What does the if __name__ == "__main__": do?

# Threading example
import time, thread
def myfunction(string, sleeptime, lock, *args):
    while 1:
        lock.acquire()
        time.sleep(sleeptime)
        lock.release()
        time.sleep(sleeptime)
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))
Also, what does *args mean in this example?


Mayank Tripathi
Mayank Tripathi

Total Post:397

Points:3117
Posted on    May-13-2015 6:39 AM

When the Python interpreter reads a source file, it executes all of the code found in it. Before executing the code, it will define a few special variables. For example, if the python interpreter is running that module (the source file) as the main program, it sets the special __name__ variable to have a value "__main__". If this file is being imported from another module, __name__ will be set to the module's name.

In the case of your script, let's assume that it's executing as the main function, e.g. you said something like

python threading_example.py
on the command line. After setting up the special variables, it will execute the import statement and load those modules. It will then evaluate the def block, creating a function object and creating a variable called myfunction that points to the function object. It will then read the if statement and see that __name__ does equal "__main__", so it will execute the block shown there.

One of the reasons for doing this is that sometimes you write a module (a .py file) where it can be executed directly. Alternatively, it can also be imported and used in another module. By doing the main check, you can have that code only execute when you want to run the module as a program and not have it execute when someone just wants to import your module and call your functions themselves

Don't want to miss updates? Please click the below button!

Follow MindStick