[Python-Dev] Problem with super() usage (original) (raw)

Guido van Rossum guido at python.org
Mon Jul 17 19:25:09 CEST 2006

On 7/16/06, Greg Ewing <greg.ewing at canterbury.ac.nz> wrote:

For about the third time in my life, I thought I might have found a use for cooperative super calls, but I've run into another problem with the concept.

Consider: class A(object): def m(self): print "A.m" class B(object): def m(self): print "B.m" super(B, self).m() class C(B, A): def m(self): print "C.m" super(C, self).m() >>> c = C() >>> c.m() C.m B.m A.m Okay so far, but... what if I want to use class B on its own, or in combination with other classes that don't have an m() method? >>> b = B() >>> b.m() B.m Traceback (most recent call last): File "", line 1, in ? File "", line 4, in m AttributeError: 'super' object has no attribute 'm' In general, how is one supposed to use super() in a class which may or may not be at the end of the mro with respect to a particular method?

One isn't. In the world where cooperative multiple inheritance originated (C++), this would be a static error. You can only use super when you're overriding a method, not when you're defining a new method.

To do what you want to do in such a world, you'd have to create a base class with a dummy implementation and inherit from that. You can do that in Python, too.

The only thing I can think of is to write all my super()-using methods defensively like this:

def m(self): ... s = super(C, self) if hasattr(s, 'm'): s.m() which seems awfully tedious.

Definitely don't that.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

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