Dirk Thierbach <[EMAIL PROTECTED]> wrote: > Ok. Multiple inheritance can often select priority for conflicting > methods. If you can specify yhat tou want "column priority" for > each class, you're fine.
On second thought, I had doubts. Consider the following scenario: A2 | f <- A3 ^ ^ | | B2 | f <- B3 ^ ^ | | C2 <- C3 | g Assume g calls f. Since f is defined in B version 2, it should taken over unchanged into B version 3, and that should be the version g calls. However, with multiple inheritance doing depth-first search, giving the "upper" class priority over the "left", the first path searched is B3 - A3 - A2, and hence it finds f at A2. A quick test confirms this for old-style classes, which failed in exactly that way. But with new-style classes, it works, and finds f at B2 instead. So I dug through the documentation and found that new-style classes compute a monotonic linearization of the inheritance graph, observing local precedence order, using the algorithm also used in Dylan described here: http://www.webcom.com/haahr/dylan/linearization-oopsla96.html And this algorithm will even work correctly if one leaves out B3 in the example above, inheriting directly as in C3(A3,C2), because the ordering constraint that A2 comes before B2 will still be valid in the linearization for C3. However, in the following case (shortening the notation even more) it will fail, if a method defined at C3 is looked up at D4: A1 - A2 - A3 - A4 - ... | | | | B1 - B2 - + - B4 - ... | | | | C1 - + - C3 - + - ... | | | | D1 - D2 - + - D4 - ... | | | | The solution is simply to include C3 in the list of parents of D4, as in D4(C3,B4,D2). So for every hole in a column, you have to include the first class (or classes, if the hole spans multiple rows) to the left of the hole as parents if the class just below the hole, in order from bottom to top. This adds the missing constraints, and should solve the problem without any need to write __metaclass__ stuff. Interesting question. I learned a lot while thinking about that. - Dirk -- http://mail.python.org/mailman/listinfo/python-list
