# This program shows off a python decorator(
# which implements tail call optimization. It
# does this by throwing an exception if it is
# it's own grandparent, and catching such
# exceptions to recall the stack.
import sys
class TailRecurseException:
 def __init__(self, args, kwargs):
  self.args = args
  self.kwargs = kwargs
def tail_call_optimized(g):
 """
 This function decorates a function with tail call
 optimization. It does this by throwing an exception
 if it is it's own grandparent, and catching such
 exceptions to fake the tail call optimization.
 This function fails if the decorated
 function recurses in a non-tail context.
 """
 def func(*args, **kwargs):
  f = sys._getframe()
  if f.f_back and f.f_back.f_back and f.f_back.f_back.f_code == f.f_code:
   raise TailRecurseException(args, kwargs)
  else:
   while 1:
    try:
     return g(*args, **kwargs)
    except TailRecurseException, e:
     args = e.args
     kwargs = e.kwargs
 func.__doc__ = g.__doc__
 return func
@tail_call_optimized
def factorial(n, acc=1):
 "calculate a factorial"
 if n == 0:
  return acc
 return factorial(n-1, n*acc)
print factorial(10000)
# prints a big, big number,
# but doesn't hit the recursion limit.
@tail_call_optimized
def fib(i, current = 0, next = 1):
 if i == 0:
  return current
 else:
  return fib(i - 1, next, current + next)
print fib(10000)
# also prints a big number,
# but doesn't hit the recursion limit.