[Twisted-Python] Phasing out old-style classes

[Kelly]

On Sun, Oct 25, 2009 at 5:32 PM, Glyph Lefkowitz
<glyph at twistedmatrix.com> wrote:
> On Oct 25, 2009, at 1:50 AM, James Y Knight wrote:
>
>> On Oct 25, 2009, at 1:38 AM, Glyph Lefkowitz wrote:
>>> # in your application
>>> class Application(object, Library): pass
>>
>> Maybe the answer "why not" is obvious and I should already know it, but
>> wouldn't it be more obvious to write:
>> class Application(Library, object): pass
>> instead. That at least has the advantage of not blowing up if Library later
>> becomes newstyle, right?
>
> Huh.  You may have just pointed out that I misunderstand the algorithm used
> to generate that error.  I thought that certain other hierarchies in cases
> more complex than the trivial one I presented would also give you that error
> if something became new-style in the middle, but I can't seem to generate
> any hierarchies that cause a problem.  Is it really this simple?  Always put
> 'object' at the *end* of your bases-list to declare the intention "I want to
> be new-style" and you'll be OK?  (If so, why is that?)

>From my understanding of the C3 algorithm, the relative order of
classes in a base class's mro will be preserved in classes that
inherit from it.  Since "object" inherits from nothing, it can always
be shunted to the end of the mro, provided it hasn't ever been
declared to be ahead of anything else.