Python 是否优化尾递归?

2022-01-22 00:00:00 python 递归 stack-overflow stack 尾部递归

问题描述

我有以下代码失败并出现以下错误:

I have the following piece of code which fails with the following error:

RuntimeError: 超出最大递归深度

RuntimeError: maximum recursion depth exceeded

我试图重写它以允许尾递归优化 (TCO).我相信如果发生了 TCO，这段代码应该是成功的.

I attempted to rewrite this to allow for tail recursion optimization (TCO). I believe that this code should have been successful if a TCO had taken place.

def trisum(n, csum):
    if n == 0:
        return csum
    else:
        return trisum(n - 1, csum + n)

print(trisum(1000, 0))

我应该断定 Python 不会产生任何类型的 TCO，还是只需要以不同的方式定义它?

Should I conclude that Python does not do any type of TCO, or do I just need to define it differently?

解决方案

没有，以后也不会了 Guidovan Rossum 更喜欢能够有适当的回溯:

No, and it never will since Guido van Rossum prefers to be able to have proper tracebacks:

尾递归消除 (2009-04-22)

Tail Recursion Elimination (2009-04-22)

尾调用的最后一句话 (2009-04-27)

Final Words on Tail Calls (2009-04-27)

您可以通过如下转换手动消除递归:

You can manually eliminate the recursion with a transformation like this:

>>> def trisum(n, csum):
...     while True:                     # Change recursion to a while loop
...         if n == 0:
...             return csum
...         n, csum = n - 1, csum + n   # Update parameters instead of tail recursion

>>> trisum(1000,0)
500500

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