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

Before executing code, Python interpreter reads source file and define few special variables/global variables.
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. Module’s name is available as value to __name__ global variable.
A module is a file containing Python definitions and statements. The file name is the module name with the suffix .py appended.
When we execute file as command to the python interpreter,

python script.py
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# Python program to execute 
# main directly
print "Always executed"
  
if __name__ == "__main__":
    print "Executed when invoked directly"
else:
    print "Executed when imported"

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  • All of the code that is at indentation level 0 [Block 1] gets executed. Functions and classes that are defined are, well, defined, but none of their code runs.
  • Here, as we executed script.py directly __name__ variable will be __main__. So, code in this if block[Block 2] will only run if that module is the entry point to your program.
  • Thus, you can test whether your script is being run directly or being imported by something else by testing __name__ variable.
  • If script is getting imported by some other module at that time __name__ will be module name.

Why Do we need it?

For example we are developing script which is designed to be used as module:



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# Python program to execute 
# function directly
def my_function()
      print "I am inside function"
  
# We can test function by calling it.
my_function()

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Now if we want to use that module by importing we have to comment out our call. Rather than that approach best approach is to use following code:

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# Python program to use 
# main for function call.
if __name__ == "__main__":
    my_function()
  
import myscripy
  
myscript.my_function()

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Advantages :

  1. Every Python module has it’s __name__ defined and if this is ‘__main__’, it implies that the module is being run standalone by the user and we can do corresponding appropriate actions.
  2. If you import this script as a module in another script, the __name__ is set to the name of the script/module.
  3. Python files can act as either reusable modules, or as standalone programs.
  4. if __name__ == “main”: is used to execute some code only if the file was run directly, and not imported.

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