[Python-Dev] Things to Know About Super (original) (raw)

Phillip J. Eby pje at telecommunity.com
Wed Aug 27 05:15:36 CEST 2008

At 03:16 AM 8/27/2008 +0200, Michele Simionato wrote:

It is just a matter of how rare the use cases really are. Cooperative methods has been introduced 6+ years ago. In all this time surely they must have been used. How many compelling uses of cooperation we can find in real life code? For instance in the standard library or in some well known framework? This is a serious question I have been wanting to ask for years. I am sure people here can find some example, so just give me a pointer and we will see.

ISTR pointing out on more than one occasion that a major use case for co-operative super() is in the implementation of metaclasses. The init and new signatures are fixed, multiple inheritance is possible, and co-operativeness is a must (as the base class methods must be called). I'm hard-pressed to think of a metaclass constructor or initializer that I've written in the last half-decade or more where I didn't use super() to make it co-operative.

That, IMO, is a compelling use case even if there were not a single other example of the need for super. However, I'm pretty sure I've had other cases where it was necessary to co-operate in cases where multiple inheritance occurred later; ie. where it was possible for a subclass to add a new class between parents. Remember that subclasses of a new-style class do not always have the same MRO tail as the original class; i.e., a subclass of "class A(B, C):" is only constrained to have [A...B...C] in its MRO; semi-arbitrary classes may be inserted between e.g. A and B. So, a new-style class cannot, as a general rule, statically determine what base class implementation of a method should be invoked. I personally consider the rare case where I have to force such static knowledge to be an unfortunate wart in the design (of that code, not Python).

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