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Help with an algorithm wanted: a solution (improvements gratefully accepted)

Russell E. Owen rowen at cesmail.net
Thu Jun 27 19:57:19 EDT 2002 
In article <rowen-CE3C0F.13020226062002 at nntp1.u.washington.edu>,
 "Russell E. Owen" <rowen at cesmail.net> wrote a long complicated message
about a generating what is basically a data converter object.

I really appreciate everybody's help on this. I apologize for not making 
the problem clearer in the first place.

I offer my solution below with enough comments to, I hope, make the 
problem clearar. Any critique would be appreciated.

"""Sample object that converts between different data
representations A-D, starting from a given value in one representation.

In reality there will be more representations and the conversion code
will be complicated (though much of it may reside in an external
library of subroutines). But the graph of data representations
connected by conversion functions will always be a constant
and will always be very simple (one hub with lines of varying
numbers of nodes radiating from it).

Figuring out which order to do the conversions (as a function of
what is a given) is solved with a rather simplistic algorithm
that tests too many cases but is short and easy to read.
Thanks to Bjorn Pettersen for his help (but the crudeness is mine).

The main thing I am worried about is: given the order in which to do
the conversions, how best to create an object that implements it.
The solution shown here works, but I suspect there's a better way.
It feels as if I'm creating some sort of meta-object
(by using a function dictionary) instead of making best use
of the existing optimized object mechanism.
"""
class CnvObj (object):
    def __init__(self, givenValue, whatGiven):
        assert whatGiven in ("A", "B", "C", "D")
        self.givenValue = givenValue
        self.funcDict = {whatGiven: self.getGiven}
        self.addFunctions(whatGiven)
    
    def addFunctions(self, fromWhat):
        for toWhat in ("A", "B", "C", "D"):
            if not self.funcDict.has_key(toWhat):
                funcName = "%sfrom%s" % (toWhat, fromWhat)
                if hasattr(self, funcName):
                    self.funcDict[toWhat] = getattr(self, funcName)
                    self.addFunctions(toWhat)
    
    def getItem(self, item):
        return self.funcDict[item]()

    def BfromC(self):
        return "BfromC(%s)" % self.getItem("C")
    
    def CfromA(self):
        return "CfromA(%s)" % self.getItem("A")
    
    def DfromC(self):
        return "DfromC(%s)" % self.getItem("C")
    
    def getGiven(self):
        return self.givenValue
    
    #...etc. only the functions needed for "A given" are shown

co = CnvObj("value for A", "A")
for rep in ("A", "B", "C", "D"):
    print co.getItem(rep)

"""this prints out:
value for A
BfromC(CfromA(value for A))
CfromA(value for A)
DfromC(CfromA(value for A))
"""
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