Paragraph Matching in Python

So I developed some code as part of a larger project. I came upon a problem with how to match paragraphs, and wasn't sure how to proceed, so I asked on Stack Overflow here. You can find an in-depth description of my problem there if you're curious.

Just to be clear, I am not reposting the same question here to get an answer.

I came up with a solution to my own problem, but I'm unsure of limits/pitfalls, and here seems like the perfect place for that.

The short version on the explanation is this: I have two strings, one is the revised version of the other. I want to generate markups and preserve the paragraph spacing, thus, I need to correlate the list of paragraphs in each, match them, and then mark the remaining as either new or deleted.

So I have a function (paraMatcher()) which matches paragraphs and returns a list of tuples as follows:

• (num1, num2) means that the best match for revised paragraph num1 is original paragraph num2

• (num, '+') means that there is no match for revised paragraph num, so it must be new (designated by the '+')

• (num, '-') means that no revised paragraph was matched to original paragraph num so it must have been deleted (designated by the '-')

So without further adiue, here is my function:

def paraMatcher(orParas, revParas):  
    THRESHOLD = 0.75
    matchSet = []
    shifter = 0

    for revPara in revParas:
        print "Checking revPara ", revParas.index(revPara)
        matchTuples = [(difflib.SequenceMatcher(a=orPara,b=revPara).ratio(), orParas.index(orPara)) for orPara in orParas]
        print "MatchTuples: ", matchTuples
        if matchTuples:
            bestMatch = sorted(matchTuples, key = lambda tup: tup[0])[-1]
            print "Best Match: ", bestMatch
            if bestMatch[0] > THRESHOLD:
                orParas.pop(bestMatch[1])
                print orParas
                matchSet.append((revParas.index(revPara), bestMatch[1] + shifter))
                shifter += 1
            else:
                matchSet.append((revParas.index(revPara), "+"))

        print ""
        print "---------------"
        print ""

    if orParas:
        print "++++++++++++++dealing with extra paragraphs++++++++++++++"
        print orParas
        for orPara in orParas:
            matchSet.insert(orParas.index(orPara) + shifter, (orParas.index(orPara) + shifter, "-"))

    return matchSet 

While I definitely want review of general coding style, etc, here are a few issues that I'm really interested in getting feedback on:

• The function needs to be called with copies of the lists (paraMatcher(lst1[:], lst2[:]))
• How might this fail?
• How do I determine an appropriate value for THRESHOLD

Some Extra Notes:

• I've left the diagnostic printing in there in case any of you want to test it
• Due to other parts of the code, its more convenient that this function take lists of paragraphs as arguments, rather than the strings
• I don't think it matters, but this is 32-bit Python 2.7.6 running on 64-bit Windows 7