Python中的尾递归优化装饰器

Question

我最近一直在学习Scala，所以我用Python编写了一些递归。

而且我发现Python中没有尾递归优化。

然后，我发现了一个魔术器（？）装饰器 ，该装饰器似乎可以优化尾递归。

它解决了RuntimeError: maximum recursion depth exceeded 。

但是我不明白这段代码的工作方式和原因。

有人可以解释这段代码中的神奇力量吗？

码：

# This program shows off a python decorator(
# which implements tail call optimization. It
# does this by throwing an exception if it is 
# its 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 its 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.

Answer 1

没有尾调用优化，您的堆栈如下所示：

factorial(10000)
factorial(9999)
factorial(9998)
factorial(9997)
factorial(9996)
...

并一直增长到您到达sys.getrecursionlimit()调用（然后是kaboom）为止。

带有尾部呼叫优化：

factorial(10000,1)
factorial(9999,10000) <-- f.f_back.f_back.f_code = f.f_code? nope
factorial(9998,99990000) <-- f.f_back.f_back.f_code = f.f_code? yes, raise excn.

并且该异常使装饰器转到其while循环的下一个迭代。