ssecorp wrote:

http://aspn.activestate.com/ASPN/Coo.../Recipe/496691

so I try it and when I run:

@Decorators.tail_recursion

def fibtr(n):

def fibt(a, b, n):

if n <= 1:

return b

else:

return fibt(b, a + b, n - 1)

if n == 0:

return 0

else:

return fibt(0, 1, n);

it still blows the stack. so what is the point? is it impossible to

get "real" tail-recursion in Python?

As you have used it, the decorator wraps the *outer* non-recursive

function which is just called once anyway. Useless. Try wrapping fibt

instead.

That said, this recipe significantly increases the running time by

multiplying the number of function calls by about three. I do not

regard it as removing the recursion, but, rather, as making it indirect

(via two other calls) so as to remove the unneeded stack frames (and the

space problem) in between recursive calls. Much simpler is the trivial

rewrite with while to do 'in frame recursion', or iteration. This also

removes the need for outer and inner function.

rearrange fibt as

def fibt(a,b,n):

if n 1:

return fibt(b, a+b, n-1)

else:

return b

and rewrite as

def fibi(a,b,n):

while n 1:

a,b,n = b,a+b,n-1

return b

by directly binding the new arguments to the parameters.

Move the initialization inside the function (and delete the outer

wrapper) to get

def fib(n):

if n==0:

return 0

else:

a,b = 0,1

while n 1:

a,b,n = b,a+b,n-1

return b

and even turn the induction back a step and simplify to

def fib(n):

a,b = 1,0

while n:

a,b,n = b,a+b,n-1

return b

Why do some people fight writing efficient beautiful code like this that

works with Python's design to instead write less efficient and uglier

code that works against Python's design?

If you do not want function calls (and runtime name resolution), do not

write them!

Terry Jan Reedy