Foward Chaining expert system

mycin.py

#! /usr/bin/env python2
##
# Implementation of MyCIN-like expert system
# Copyright (c) 2012, Joseph Walton-Rivers <joseph@webpigeon.me.uk>
##

from collections import defaultdict

CV_TRUE = +1.0
CV_UNKNOWN = 0
CV_FALSE = -1.0

class Solver(object):

	def __init__(self):
		self.facts = defaultdict(lambda: defaultdict(float))
		self.rules = set()

	def run(self):
		while self.rules:
			currRule = None

			# look at rules
			for rule in self.rules:
				if self.canFire(rule):
					currRule = rule
					break

			# No rule can be executed
			if not currRule:
				break
			
			# Update the fact for this rule
			self.rules.remove(currRule)
			self.addFact(currRule[1][0], currRule[1][1], currRule[1][2])

	def canFire(self, rule):
		for (fact,value) in rule[0]:
			if fact not in self.facts:
				return False

			if value not in self.facts[fact]:
				return False
		return True

	def addFact(self, fact, value, cRule):
		cCurr = self.facts[fact][value]
		cNew = cCurr + cRule * (1 - cCurr)
		self.facts[fact][value] = cNew

def prettyPrint(goal, facts):
	items = facts[goal].items()
	items = filter(lambda x: x[1] > 0.2, items)
	items = sorted(items, key=lambda x: -x[1])

	if not items:
		print("Sorry, I couldn't figure it out")
		return

	for (p, value) in items:
		print value, p

def main():
	solver = Solver()
	solver.addFact("ears", "pointy", 1)
	solver.addFact("tail", "waggy", 1)
	solver.addFact("noise", "meow", 1)
	solver.addFact("size", "small", 1)

	solver.rules.add( ((("ears", "pointy"),), ("animal", "cat", 0.7)) )
	solver.rules.add( ((("tail", "waggy"),), ("animal", "dog", 0.5)) )
	solver.rules.add( ((("noise", "meow"),), ("animal", "cat", 0.9)) )
	solver.rules.add( ((("noise", "woof"),), ("animal", "dog", 1)) )
	solver.rules.add( ((("size", "small"),), ("animal", "cat", 0.9)) )
	solver.rules.add( ((("size", "small"),), ("animal", "dog", 0.3)) )
	solver.run()

	prettyPrint("animal", solver.facts)

if __name__ == "__main__":
	main()