If an item, for example, "Bourqoqne" appears multiple times in a given document, "Coche-Dury Bourgogne Chardonay 2005, Bourgogne, France", your algorithm will append this same item into the IrIndex.index list and IrIndex.tf list multiple times. This multiple-append implementation distorts the calculation of total number of documents containing the given item in the following code:

idf = log( float( len(self.documents) ) / float( len(self.tf[term]) ) )

I changed the code from:

for term in terms:
if term not in self.index:
self.index[term] = []
self.tf[term] = []

        self.index[term].append(document_pos)
        self.tf[term].append(terms.count(term))

to:

for term in terms:
if term not in self.index:
self.index[term] = []
self.tf[term] = []

        if document_pos not in self.index[term]:
            self.index[term].append(document_pos)
            self.tf[term].append(terms.count(term))

by skipping the subsequent append operations if an item in conjunction with its containing document is already recorded inside an IrIndex object.