Python尾递归优化

Python开启尾递归优化

cpython本身不支持尾递归优化, 但是一个牛人想出的解决办法：实现一个 tail_call_optimized 装饰器

#!/usr/bin/env python2.4
# 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:
            # 抛出异常
            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) 

这里解释一下sys._getframe()函数:

sys._getframe([depth]):
Return a frame object from the call stack.
If optional integer depth is given, return the frame object that many calls below the top of the stack.
If that is deeper than the call stack, ValueEfror is raised. The default for depth is zero,
returning the frame at the top of the call stack.

即返回depth深度调用的栈帧对象.

import sys

def get_cur_info():
    print sys._getframe().f_code.co_filename  # 当前文件名
    print sys._getframe().f_code.co_name  # 当前函数名
    print sys._getframe().f_lineno # 当前行号
    print sys._getframe().f_back # 调用者的帧

更多关于sys._getframe的使用请看Frame Hacks
说一下tail_call_optimized实现尾递归优化的原理: 当递归函数被该装饰器修饰后, 递归调用在装饰器while循环内部进行, 每当产生新的递归调用栈帧时: f.f_back.f_back.f_code == f.f_code:, 就捕获当前尾调用函数的参数, 并抛出异常, 从而销毁递归栈并使用捕获的参数手动调用递归函数. 所以递归的过程中始终只存在一个栈帧对象, 达到优化的目的.
为了更清晰的展示开启尾递归优化前、后调用栈的变化和tail_call_optimized装饰器抛异常退出递归调用栈的作用, 我这里利用pudb调试工具做了动图:

开启尾递归优化前的调用栈 

开启尾递归优化后(tail_call_optimized装饰器)的调用栈 

通过pudb右边栏的stack, 可以很清晰的看到调用栈的变化.
因为实现了尾递归优化, 所以factorial(10000)都不害怕递归深度限制报错啦!