The docs specify what argparse.FileType() does via the default parameters: https://docs.python.org/3/library/argparse.html#argparse.FileType

If you mean what does it do when you fail to call it at all (passing type=argparse.FileType), the answer is the same as any other class: It tries to construct a FileType using the user provided argument as the sole positional argument. So if the user passes a valid mode string, it works, and returns a FileType object with that mode string, otherwise it barfs and dumps an error message for passing an invalid argument for FileType.