Help with an algorithm wanted

Russell E. Owen rowen at
Wed Jun 26 16:02:02 EDT 2002

I'd like some help solving a problem in Python.

The basic idea is I have an item of data that can be represented in any 
of four ways: A,B,C and D. I also have conversion functions between 
certain pairs of representations, such that there's always a path 
between any two data representations. In this case suppose I know A<->C, 
B<->C and C<->D, or graphically:

A <--+
     +--> C <---> D
B <--+

I'd like to create objects that have one of these items (A,B,C,D) as a 
given, then be able to query any such object for any data representation.

For instance, an object with A as the given would return D by solving 

What I'm trying to figure out is a simple and reasonably elegant way to 
code such objects.

I am pretty sure recursion is the heart of the solution. An object with 
A as the given could have a method getD that returned D<-C and a method 
getC that returned C<-A.

The real issue is creating such objects without hand coding each one 
(since in the real world I have more representations and the number of 
casese proliferates badly).

I suspect I should use recursion to create the object, too, but it 
sounds like it'll need a lot of introspection. For instance to create an 
object with A as the given, one might:
- start by defining method getA to return A
- look through the set of conversion functions that output A; the only 
one is A<-C, so method getC returns C<-A.
- repeat with all methods that return C and don't have methods already 
defined; this gives method getB returns B<-C and method getD returns 

Alternatively, I can imagine making each object basically identical 
except for some kind of meta-data that tells each method which data item 
to query; the appropriate function can then be looked up as needed (e.g. 
via a dictionary that takes as a key the output and input 
representation). This would slow down the methods but avoid the 

Any suggestions?

-- Russell

More information about the Python-list mailing list