Python反向生成器 [英] Python Reverse Generator

问题描述

我正在寻找一种反转生成器对象的方法.我知道如何反转序列:

I'm looking for a way to reverse a generator object. I know how to reverse sequences:

foo = imap(seq.__getitem__, xrange(len(seq)-1, -1, -1))

但是使用生成器作为输入并使用反向生成器作为输出(len(seq)保持不变，因此可以使用原始序列的值)是否有可能类似?

But is something similar possible with a generator as the input and a reversed generator as the output (len(seq) stays the same, so the value from the original sequence can be used)?

推荐答案

除非将生成器强制转换为序列并从中创建迭代器，否则无法以任何通用方式反转生成器.在计算出较早的项之前，不一定要知道生成器的后项.

You cannot reverse a generator in any generic way except by casting it to a sequence and creating an iterator from that. Later terms of a generator cannot necessarily be known until the earlier ones have been calculated.

更糟糕的是，直到您碰到生成器，您才知道它是否会遇到StopIteration异常，因此无法知道序列中的第一个术语.

Even worse, you can't know if your generator will ever hit a StopIteration exception until you hit it, so there's no way to know what there will even be a first term in your sequence.

最好的办法是编写一个reversed_iterator函数:

The best you could do would be to write a reversed_iterator function:

def reversed_iterator(iter):
    return reversed(list(iter))

当然，您也可以使用基于imap的迭代版本替换其中的reversed，以保存一个列表创建.

You could also, of course, replace reversed in this with your imap based iterative version, to save one list creation.

这篇关于Python反向生成器的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！