On Aug 29, 2008, at 11:46 AM, Michele Simionato wrote:
On Fri, Aug 29, 2008 at 6:15 PM, Nick Coghlan <ncoghlan@gmail.com> wrote:
The mixin methods in the ABC machinery would be a lot less useful without multiple inheritance (and the collections ABCs would be a whole lot harder to define and to write).
So if you're looking for use cases for multiple inheritance, I'd suggest starting with the Python 2.6 collections module and seeing how you would go about rewriting it using only single inheritance. I believe the new io module is also fairly dependent on multiple inheritance.
I am very well aware of the collection module and the ABC mechanism. However, you are missing that mixins can be implemented in a single- inheritance world without the complications of the MRO. See my answer to Alex Martelli in this same thread.
As interesting as this conversation is at a meta-level, I'm not sure how much more can be accomplished here by debating the merits of multiple vs. single inheritance. Unfortunately I think this is a case where there is not just one good way to do it in all cases, especially given the subjective nature of "good" in this context. This is what I take away from this: - super() is tricky to use at best, and its documentation is inaccurate and incomplete. I think it should also be made more clear that super() is really mostly useful for framework developers, including users extending frameworks. Unfortunately many frameworks require you to extend them in order to write useful applications in my experience, so it trickles down to the app developer at times. In short, more correct documentation == good. - The difficulties of super() are really symptomatic of the difficulties of multiple inheritance. I think it's clear that multiple inheritance is here to stay in Python, and it solves certain classes of problems quite well. But, it requires careful consideration, and it's easy to get carried away and create a huge mess (ala Zope2, which I am all too familiar), and it can hinder code clarity as much as it facilitates reuse. - There are good alternatives to multiple inheritance for many cases, but there are cases where multiple inheritance is arguably best. Traits are a possible alternative that deserve further study. I think that study would be greatly aided by a 3rd-party library implementing traits for Python. If traits are to gain any traction or ever be considered for inclusion into the language such a library would need to exist first and demonstrate its utility. I know I'm probably just stating the obvious here, but I found it therapeutic ;^) -Casey