Decorators not worth the effort

[Chris Angelico]

On Sat, Sep 15, 2012 at 12:12 AM, andrea crotti
<andrea.crotti.0 at gmail.com> wrote:
> def fib(n):
>     if n <= 1:
>         return 1
>     return fib(n-1) + fib(n-2)
>
> @memoize
> def fib_memoized(n):
>     if n <= 1:
>         return 1
>     return fib_memoized(n-1) + fib_memoized(n-2)
>
>
> The second fibonacci looks exactly the same but while the first is
> very slow and would generate a stack overflow the second doesn't..

Trouble is, you're starting with a pretty poor algorithm. It's easy to
improve on what's poor. Memoization can still help, but I would start
with a better algorithm, such as:

def fib(n):
	if n<=1: return 1
	a,b=1,1
	for i in range(1,n,2):
		a+=b
		b+=a
	return b if n%2 else a

def fib(n,cache=[1,1]):
	if n<=1: return 1
	while len(cache)<=n:
		cache.append(cache[-1] + cache[-2])
	return cache[n]

Personally, I don't mind (ab)using default arguments for caching, but
you could do the same sort of thing with a decorator if you prefer. I
think the non-decorated non-recursive version is clear and efficient
though.

ChrisA