Integer multiplication overflow.

[johnvert at my-deja.com]

Hello,

I wrote the following simple recursive factorial function in python:

def fact(num):
  if num < 2: 1
  return num * fact(num - 1)

When I run it, I get the following:

>>> fact(3)
6

which is correct, but if I use something slightly larger:

>>> fact(15)
Traceback (innermost last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in fact
  File "<stdin>", line 3, in fact
  File "<stdin>", line 3, in fact
OverflowError: integer multiplication

Why is that?  If I compare this with the Scheme equivalent:

(define (fact num)
  (if (< num 2)
    1
    (num * (fact (- num 1)))))

and run it with MzScheme, I can do:

> (fact 15)
1307674368000

> (fact 20)
2432902008176640000

and even,

> (fact 154)
30897696138473508879585646703632404659201907040888820477871589289865505687886666220300447285640952619071680544337494109264649994680187591361311072737951454695525676891035640863743200899694758450943586711068571022031011228320107310612480000000000000000000000000000000000000

So why does python choke (pun intended) on a slightly large
multiplication?  I know it's not meant to be very numeric (although I
hear it's capable of a lot with the Numeric package,) but the factorial
of 15 is trivial.

Thanks a lot,
  -- john
Les python newbie



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