Which exception should I raise on bad/illegal argument combinations in Python?

python exception arguments
cdleary picture cdleary · Nov 2, 2008 · Viewed 253.9k times · Source

I was wondering about the best practices for indicating invalid argument combinations in Python. I've come across a few situations where you have a function like so:

def import_to_orm(name, save=False, recurse=False):
    """
    :param name: Name of some external entity to import.
    :param save: Save the ORM object before returning.
    :param recurse: Attempt to import associated objects as well. Because you
        need the original object to have a key to relate to, save must be
        `True` for recurse to be `True`.
    :raise BadValueError: If `recurse and not save`.
    :return: The ORM object.
    """
    pass

The only annoyance with this is that every package has its own, usually slightly differing BadValueError. I know that in Java there exists java.lang.IllegalArgumentException -- is it well understood that everybody will be creating their own BadValueErrors in Python or is there another, preferred method?

Answer

dbr picture dbr · Nov 2, 2008

I would just raise ValueError, unless you need a more specific exception..

def import_to_orm(name, save=False, recurse=False):
    if recurse and not save:
        raise ValueError("save must be True if recurse is True")

There's really no point in doing class BadValueError(ValueError):pass - your custom class is identical in use to ValueError, so why not use that?

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