What Does if __name__ == "__main__" Do in Python?

if __name__ == "__main__": runs the indented code only when the file is executed directly, for example with python app.py. When the same file is imported as a module, that block is skipped.

Why it works

Python sets the variable __name__ for every module. When a file is run as the top-level script, __name__ is "__main__". When it is imported, __name__ is the module's name.

Example

def main():
    print("running")

if __name__ == "__main__":
    main()

python app.py prints "running"; import app prints nothing.

When to use it

Use it for scripts that also expose reusable functions, so tests and other modules can import them without side effects. It is also needed with multiprocessing on platforms that spawn new processes.

Sources

• 01docs.python.org/3/library/__main__.html