Python main function is executed only when it’s being executed as a python program. As you know, we can also import a python program as a module, in that case python main method should not execute.
Python main function
Main function is the entry point of any program. But python interpreter executes the source file code sequentially and doesn’t call any method if it’s not part of the code. But if it’s directly part of the code then it will be executed when the file is imported as a module.
That’s why there is a special technique to define main method in python program, so that it gets executed only when the program is run directly and not executed when imported as a module. Let’s see how to define python main function in a simple program.
print("Hello") print("__name__ value: ", __name__) def main(): print("python main function") if __name__ == '__main__': main()
- When a python program is executed, python interpreter starts executing code inside it. It also sets few implicit variable values, one of them is
__name__whose value is set as
- For python main function, we have to define a function and then use
if __name__ == '__main__'condition to execute this function.
- If the python source file is imported as module, python interpreter sets the
__name__value to module name, so the if condition will return false and main method will not be executed.
- Python provides us flexibility to keep any name for main method, however it’s best practice to name it as main() method. Below code is perfectly fine, however not recommended.
def main1(): print("python main function") if __name__ == '__main__': main1()
Below image shows the output when
python_main_function.py is executed as source file.
Python main function as module
Now let’s use above python source file as a module and import in another program.
import python_main_function print("Done")
Now when above program is executed, below output is produced.
Hello __name__ value: python_main_function Done
Notice that first two lines are getting printed from
python_main_function.py source file. Notice the value of
__name__ is different and hence main method is not executed.
Notice that python program statements are executed line by line, so it’s important to define the main() method first before the if condition to execute main method. Otherwise you will get error as
NameError: name 'main' is not defined.
That’s all about python main function.
Reference: Python Docs