How to create a return flag object

Question

Currently a set of functions returns success=True or False.

We've discovered this isn't good enough, though, since False can convey both "valid result" or "invalid result", and we want behavior to differ in each case.

So I think they should be changed to instead return {True, False, InvalidResult}, where bool(InvalidResult) is false for backward compatibility, but can be tested for using if is InvalidResult.

I'm not sure what the terminology is, but I'm imagining something like the built-in NotImplemented that's returned by comparison functions. This is called a "special value" in the docs and is of type NotImplementedType.

How to create such an object and what methods/attributes should it have? I should create my own type like NotImplementedType also, or is there an existing type that conveys this "flag" concept? It's a similar kind of object to True, False, None, NotImplemented, etc.

Sounds like a "tribool"... Is `None` also a valid value or could that be used to mean no result? — Jon Clements, Oct 15 '17 at 17:34
@georgexsh Yes, but it was thought that InvalidResult is clearer — endolith, Oct 15 '17 at 17:35
@georgexsh Added a link https://github.com/scipy/scipy/pull/7819#issuecomment-331761473 — endolith, Oct 15 '17 at 17:37
@endolith okay... so `InvalidResult = None` - and you're done? :p — Jon Clements, Oct 15 '17 at 17:40
@endolith You could have the invalid result raise an exception. Or a (success, valid) tuple. — Roland Smith, Oct 15 '17 at 17:42
@JonClements No, that doesn't work. Printing the return value would be empty, etc. — endolith, Oct 15 '17 at 17:42
@endolith then you'd need to create your own singleton with desired behaviour or special case `if result is InvalidResult` (print something else...) — Jon Clements, Oct 15 '17 at 17:43
@JonClements Yes, that sounds right. So it's called a "singleton"? How do you create one? — endolith, Oct 15 '17 at 17:45
@endolith seek and ye' will find... plenty of examples around the place by using singleton in conjuction with Python and "how do I make"... :) — Jon Clements, Oct 15 '17 at 17:47
It seems that raising an exception and handling that may be the more straightforward way of dealing with the invalid input, even if you just pass on the exception — Stuart Buckingham, Oct 15 '17 at 17:54
@StuartBuckingham Well the function returns a dictionary of "here's what I tried and this is how it went", of which `success={True, False}` is just one component, so I don't think halting everything and raising an exception is the way to do it. — endolith, Oct 15 '17 at 18:09
@JonClements Yes, [Tribool](http://www.grantjenks.com/docs/tribool/) sounds like the same concept — endolith, Oct 15 '17 at 18:13
(Normally I'd just have it return a string, but `bool('a string')` is True) — endolith, Oct 15 '17 at 18:26
Interesting PEP to this topic: [PEP 661 -- Sentinel Values](https://www.python.org/dev/peps/pep-0661/) — Czechnology, Jul 31 '21 at 21:44

Matthias Fripp · Answer 1 · 2017-10-15T17:59:52.903

You could just use None or 0 as the InvalidResult value, e.g. in my_mod, define InvalidResult = None, then elsewhere you can test if result is my_mod.InvalidResult. See here for some more info on the "truthfulness" of None: False or None vs. None or False

Or you could define an object with suitable methods for Boolean conversion; hopefully others will chime in with those details.

Note that whichever way you go, you'll have to be careful if you have multipart Boolean expressions: InvalidResult and False will give InvalidResult but False and InvalidResult will give False.

score 1 · Answer 2 · answered Oct 16 '17 at 00:41

Apparently this is called a "sentinel" and is a simple as this:

class InvalidResultType(object):
    """
    Indicates that minimization has failed and result is invalid (such as a
    boundary or constraint violation)
    """
    def __repr__(self):
        return 'InvalidResult'
    def __bool__(self):
        return False
    def __reduce__(self):
        return 'InvalidResult'

InvalidResult = InvalidResultType()


success = InvalidResult

assert success == InvalidResult
assert success is InvalidResult
assert not bool(InvalidResult)
assert InvalidResult != True
assert InvalidResult != False  # Not sure about this yet
assert InvalidResult != None

Now of course I find the similar questions:

Defining my own None-like Python constant

and the __reduce__ might be overkill; I'm not sure if pickling or copying will ever matter

How to create a second None in Python? Making a singleton object where the id is always the same

How to create a return flag object

2 Answers2