[Python-Dev] Re: super() harmful? (original) (raw)

James Y Knight foom at fuhm.net
Wed Jan 5 17:55:54 CET 2005

I'm not sure why super got dragged into this, but...

On Jan 4, 2005, at 9:02 PM, Guido van Rossum wrote:

I think that James Y Knight's page misrepresents the issue. Quoting:

But init is special, in that it is okay for a subclass init (or new) to have a different signature than the base class init; this is not true for other methods. If you change a regular method's signature, you would break Liskov substitutability (i.e., your subclass instance wouldn't be acceptable where a base class instance would be acceptable).

You're right, some issues do apply to init alone. However, two important ones do not:

The issue of mixing super() and explicit calls to the superclass's method occur with any method. (Thus making it difficult/impossible for a framework to convert to using super without breaking client code that subclasses).

Adding optional arguments to one branch of the inheritance tree, but not another, or adding different optional args in both branches. (breaks unless you always pass optional args as keywordargs, and all methods take **kwargs and pass that on to super).

Super is intended for use that are designed with method cooperation in mind, so I agree with the best practices in James's Conclusion: [[omitted]] But that's not the same as calling it harmful. :-(

The 'harmfulness' comes from people being confused by, and misusing super, because it is so very very easy to do so, and so very hard to use correctly.

From what I can tell, it is mostly used incorrectly. Especially uses in init or new. Many people seem to use super in their init methods thinking that it'll magically improve something (like perhaps making multiple inheritance trees that include their class work better), only to just cause a different set of problems for multiple inheritance trees, instead, because they don't realize they need to follow those recommendations.

Here's another page that says much the same thing, but from the viewpoint of recommending the use of super and showing you all the hoops to use it right: http://wiki.osafoundation.org/bin/view/Chandler/UsingSuper

James

PS, I wrote that page last pycon but never got around to finishing it up and therefore never really publically announced it. But I told some people about it and then they kept asking me for the URL so I linked to it, and well, then google found it of course, so I guess it's public now. ;)

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