Help with an algorithm wanted

[Niklas Frykholm]

[Russell E. Owen]:
> 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.

Here is one possible solution. Given a set of classes A, B, C... and
known conversion functions A.to_B, B.to_C, etc this code creates all
derived conversion functions (such as A.to_C). It thus solves the problem
statically. (In some cases a dynamic solution is preferrable.)

	def add_conversion(fromm, via, to):
		"Add conversion from class fromm to class to, via class via."
		def convert(self):
			via_object = getattr(self, "to_" + via)()
			return getattr(via_object, "to_" + to)()
		setattr(globals()[fromm], "to_" + to, convert)

	def has_conversion(fromm, to):
		"Is there a conversion function from class fromm to class to?"
		return hasattr(globals()[fromm], "to_" + to)

	def conversions(klass):
		"Returns all the conversion functions of klass."
		l = filter(lambda s: s.startswith('to_'), dir(globals()[klass]))
		return map(lambda s: s[3:len(s)], l)

	def create_quick_conversions(classes):
		"Create all possible derived conversions for the set of classes."
		for via in classes:
			tos = conversions(via)
			for fromm in classes:
				if fromm != via and has_conversion(fromm, via):
					for to in tos:
						if not has_conversion(fromm, to):
							add_conversion(fromm, via, to)
		# end of for

	create_quick_conversions(["A", "B", "C", "D"])

// Niklas