Question

I have this code for computing fibonacci numbers using cache (dictionary).

cache = {} 
def dynamic_fib(n):
    print n
    if n == 0 or n == 1:
        return 1
    if not (n in cache):
        print "caching %d" % n
        cache[n] = dynamic_fib(n-1) + dynamic_fib(n-2)

    return cache[n]

if __name__ == "__main__":
    start = time.time()
    print "DYNAMIC: ", dynamic_fib(2000)
    print (time.time() - start)

I works fine with small numbers, but with more than 1000 as an input, it seems to stop.

This is the result with 2000 as an input.

....
caching 1008
1007
caching 1007
1006
caching 1006
1005
caching 1005

This is a result with 1000 as an input.

....
8
caching 8
7
caching 7
6
caching 6
5
caching 5

It looks like that after 995 storage into the dictionary, it just hangs.
What might be wrong in this? What debugging technique can I use to see what went wrong in python?

I run python on Mac OS X 10.7.5, I have 4G bytes of RAM, so I think some KB (or even MB) of memory usage doesn’t matter much.

Answer 1

Python has a default recursion limit set to 1000.
You need to increase it in your program.

import sys

sys.setrecursionlimit(5000)

From : http://docs.python.org/2/library/sys.html#sys.setrecursionlimit

sys.setrecursionlimit(limit)

Set the maximum depth of the Python interpreter stack to limit. 
This limit prevents infinite recursion from causing an overflow of the C 
stack and crashing Python.
The highest possible limit is platform-dependent. A user may need to set the
limit higher when she  has a program that requires deep recursion and a 
platform that supports a higher limit. This should bedone with care, because
a too-high limit can lead to a crash.