stylistic question -- optional return value

Erik Max Francis max at alcyone.com
Wed Aug 28 21:37:48 CEST 2002


Andrew Koenig wrote:

> Another possibility is to return either (x, None) or (x, y).  Now
> it is easy to extract x from the compound result.  However,
> that strategy removes None from the set of permissible values for y.

If that's a problem, you can always use the fake object trick:

	class C:
	    Sentinel = object()
	    def soAndSo(self, ...):
	        return x, C.Sentinel

You're guaranteed that C.Sentinel will be a unique object, distinct from
None or any other object you might want to return, so you can do a test
like

	x, y = c.soAndso(...)
	if y is C.Sentinel:
	    # only got the one x object.

I tend to highly prefer using None for this purpose; I can't think of an
instance when I've needed to use this technique, but it is there.

> Yet another possibility is to return (False, x) or (True, x, y).
> Now x is in a common position, so retrieving it is straightforward.
> However, I can obtain the same information content by returning
> (x,) or (x, y).  However, I can easily imagine people becoming
> confused by 1-tuples.

Returning either a 1-tuple or a 2-tuple seems reasonable if what's being
returned really does have some sense of a sequence, even if it might be
a little strained.  If instead what you're really trying to return is
either one or two independent bits of information simultaneously, then
I'd say always returning a 2-tuple with either None or a proxy sentinel
object for the second element is a better approach.

-- 
 Erik Max Francis / max at alcyone.com / http://www.alcyone.com/max/
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 A lambda calculus explorer in Python.



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