Main top-level script environment

What is it

__main__ is the name of the scope in which top-level code executes. A module’s name is set equal to __main__ when read from standard input, a script, or from an interactive prompt.

A module can discover whether it is running in the main scope by checking its own __name__, which allows a common idiom for conditionally executing code in a module. When it is run as a script or with python -m but not when it is imported:

>>> if __name__ == "__main__":
...     # execute only if run as a script
...     main()

For a package, the same effect can be achieved by including a module, the contents of which will be executed when the module is run with -m.

For example, we are developing a script designed to be used as a module, we should do:

>>> def add(a, b):
...     return a+b
>>> if __name__ == "__main__":
...     add(3, 5)


  1. Every Python module has it’s __name__ defined and if this is __main__, it implies that the module is 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 is run directly, and is not being imported.

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