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:

# __name__ == "__main__": check if script is run directly (not imported)
if __name__ == "__main__":  # True when run as script, False when imported
    # execute only if run as a script
    main()

For a package, the same effect can be achieved by including a main.py 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:

# Example: function can be imported, but test code only runs when executed directly
def add(a, b):
    return a+b

if __name__ == "__main__":  # Only runs when file is executed, not when imported
    add(3, 5)

Advantages

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.

Relevant links

• Functions
• Packaging
• Python projects with Poetry and VSCode. Part 1
• import()