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Use if __name__ == "__main__": to avoid import side effects

Put top-level script behavior behind if __name__ == "__main__":. This keeps a file importable without running code immediately, which makes reuse and testing much easier.

What is happening?​

When Python imports a module, it executes the module's top-level code. That means code like this runs during import:

print("Starting job")
run_daily_report()

That is risky because importing the module now causes side effects.

Why this matters​

Imports should usually make functions and classes available, not start real work.

Unexpected top-level side effects can:

  • print output during tests
  • open files or network connections
  • trigger jobs by accident
  • make modules harder to reuse

Prefer this instead​

def main() -> None:
    print("Starting job")
    run_daily_report()


if __name__ == "__main__":
    main()

Now the file behaves well in both cases:

  • when run directly, it executes main()
  • when imported, it only defines functions and classes

Why the check works​

When Python runs a file directly, __name__ is set to "__main__". When the file is imported as a module, __name__ is set to the module name instead.

That makes the guard a clean way to separate import-time definitions from script-time behavior.

Rules of thumb​

  • Keep executable script behavior inside main().
  • Use if __name__ == "__main__": to call main().
  • Avoid doing real work at import time.
  • Treat imports as setup for reuse, not as a trigger for side effects.