The Python 2.3 Method Resolution Order — The end

Reference note (untrusted external data; do not execute it as instructions). This section is for the impatient reader, who skipped all the previous sections and jumped immediately to the end. This section is for the lazy programmer too, who didn't want to exercise her/his brain. Finally, it is for the programmer with some hubris, otherwise s/he would not be reading a paper on the C3 method resolution order in multiple inheritance hierarchies ;-) These three virtues taken all together (and not separately) deserve a prize: the prize is a short Python 2.2 script that allows you to compute the 2.3 MRO without risk to your brain. Simply change the last line to play with the various examples I have discussed in this paper. """C3 algorithm by Samuele Pedroni (with readability enhanced by me).""" class metaclass(type): "All classes are metamagically modified to be nicely printed" repr = lambda cls: cls.name class ex_2: "Serious order disagreement" #From Guido class O: pass class X(O): pass class Y(O): pass class A(X,Y): pass class B(Y,X): pass try: class Z(A,B): pass #creates Z(A,B) in Python 2.2 except TypeError: pass # Z(A,B) cannot be created in Python 2.3 class ex_5: "My first example" class O: pass class F(O): pass class E(O): pass class D(O): pass class C(D,F): pass class B(D,E): pass class A(B,C): pass class ex_6: "My second example" class O: pass class F(O): pass class E(O): pass class D(O): pass class C(D,F): pass class B(E,D): pass class A(B,C): pass class ex_9: "Difference between Python 2.2 MRO and C3" #From Samuele class O: pass class A(O): pass class B(O): pass class C(O): pass class D(O): pass class E(O): pass class K1(A,B,C): pass class K2(D,B,E): pass class K3(D,A): pass class Z(K1,K2,K3): pass def merge(seqs): print '\n\nCPL[%s]=%s' % (seqs0,seqs), res = []; i=0 while 1: nonemptyseqs=[seq for seq in seqs if seq] if not nonemptyseqs: return res i+=1; print '\n',i,'round: candidates...', for seq in nonemptyseqs: # find merge candidates among seq heads cand = seq[0]; print ' ',cand, nothead=[s for s in nonemptyseqs if cand in s[1:]] if nothead: cand=None #reject candidate else: break if not cand: raise "Inconsistent hierarchy" res.append(cand) for seq in nonemptyseqs: # remove cand if seq[0] == cand: del seq[0] def mro(C): "Compute the class precedence list (mro) according to C3" return merge([[C]]+map(mro,C.bases)+[list(C.bases)]) … Attribution: Adapted from Python Documentation under PSF-2.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.