尾递归，

sys.getrecursionlimit()

sys.setrecursionlimit()

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