def factorial(n, x):
  if n == 0:
    return x
  else:
    return factorial(n-1, n*x)

print factorial(5, 1) # 120

(n,b1=1,b2=1,c=3):
 if n<3:
  return 1
 else:
  if n==c:
   return b1+b2
  else:
   return Fib(n,b1=b2,b2=b1+b2,c=c+1)

>>> def Fib(n,b1=1,b2=1,c=3):

...  if n<3:

...   return 1

...  else:

...   if n==c:

...    return b1+b2

...   else:

...    return Fib(n,b1=b2,b2=b1+b2,c=c+1)

... 

>>> Fib(1001)

70330367711422815821835254877183549770181269836358732742604905087154537118196933579742249494562611733487750449241765991088186363265450223647106012053374121273867339111198139373125598767690091902245245323403501L

>>> 

 .....

 File "<stdin>", line 8, in Fib

 File "<stdin>", line 8, in Fib

 File "<stdin>", line 8, in Fib

 File "<stdin>", line 8, in Fib

 File "<stdin>", line 8, in Fib

 File "<stdin>", line 8, in Fib

RuntimeError: maximum recursion depth exceeded

>>> 

@tail_call_optimized
def Fib(n,b1=1,b2=1,c=3):
 if n<3:
  return 1
 else:
  if n==c:
   return b1+b2
  else:
   return Fib(n,b1=b2,b2=b1+b2,c=c+1)

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