> Apart from the call to write instead of print everything else there is a standard Python pattern for writing testable, type-safe arg-parsing code!

Its…not, actually. A more typical, and testable, example of python would be:

  import sys

  def main(args: list[str]) -> None:
    sys.stdout.write("hi\n")

  if __name__ == "__main__":
    main(sys.argv)

Your version:

* Isn’t testable because it runs the sys.exit() call unconditionally on import, so there is no way to import the module and call main() from test code.

* Fails with an error, because main has a required argument that it isn’t supplied in the unconditional main() call inside the sys.exit() call.

* Isn’t following a typical python idiom by returning a value from a function and have it be unconditionally 0; if the only options are a fixed return and a exception somewhere, the convention is to return None (which doesn’t require an explicit return.)

* Since a normal exit is the default response of ending the main program, sys.exit() is unnecessary here; “Exit with 0 if main() returns successfully, and with an abnormal exit on exceptions in main” is achieved by just calling main() and having nothing after it in the main program.