Lisp to Python translation criticism?

John E. Barham jbarham at jbarham.com
Sat Aug 17 02:59:33 CEST 2002


Don't know how many saw the story on Slashdot about Paul Graham's article
(http://www.paulgraham.com/spam.html) on how he filters spam.  He posted two
snippets of code in Lisp, a language which I only have a very passing
knowledge of.  Here's my attempt at translating it into Python:

Lisp:

(let ((g (* 2 (or (gethash word good) 0)))
      (b (or (gethash word bad) 0)))
   (unless (< (+ g b) 5)
     (max .01
          (min .99 (float (/ (min 1 (/ b nbad))
                             (+ (min 1 (/ g ngood))
                                (min 1 (/ b nbad)))))))))

(let ((prod (apply #'* probs)))
  (/ prod (+ prod (apply #'* (mapcar #'(lambda (x) (- 1 x))
                                     probs)))))

Python:

def spam_word_prob(word, good, bad, ngood, nbad):
    g = 2 * good.get(word, 0)
    b = bad.get(word, 0)
    if g + b >= 5:
        return max(0.01, min(0.99, float(min(1, b / nbad) / ((min(1, g /
ngood) + min(1, b / nbad))))))
    else:
        return 0.0

def spam_prob(probs):
    prod = 1.0
    for prob in probs:
        prod = prod * prob
    inv_probs = [1 - x for x in probs]
    inv_prob = 1.0
    for prob in inv_probs:
        inv_prob = inv_prob * prob
    return prod / (prob + inv_prob)

Any comments on the correctness, style, efficiency etc. of my translation?
I'd like to write a Python spam filtering system using Graham's techniques.

Please note that this is not meant to revive the perpetual debate over the
relative merits of Python's lambda...  ;)

    John





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