python Python反向生成器
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Python Reverse Generator
提问by ak.
I'm looking for a way to reverse a generator object. I know how to reverse sequences:
我正在寻找一种方法来反转生成器对象。我知道如何反转序列：
foo = imap(seq.__getitem__, xrange(len(seq)-1, -1, -1))
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)?
但是，将生成器作为输入，将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中的值）是否有类似的可能？
回答by jcdyer
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.
您不能以任何通用方式反转生成器，除非将其转换为序列并从中创建迭代器。在计算了较早的项之前，不一定知道生成器的后续项。
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.
更糟糕的是，您无法知道生成器是否会在遇到 StopIteration 异常之前遇到它，因此无法知道序列中的第一项。
The best you could do would be to write a reversed_iterator function:
你能做的最好的事情是编写一个 reversed_iterator 函数：
def reversed_iterator(iter):
return reversed(list(iter))
EDIT: You could also, of course, replace reversed in this with your imap based iterative version, to save one list creation.
编辑：当然，您也可以用基于 imap 的迭代版本替换这里的 reversed，以保存一个列表创建。
回答by Steve Losh
reversed(list(input_generator))is probably the easiest way.
reversed(list(input_generator))可能是最简单的方法。
There's no way to get a generator's values in "reverse" order without gathering all of them into a sequence first, because generating the second item could very well rely on the first having been generated.
如果不首先将所有生成器的值收集到一个序列中，就无法以“反向”顺序获取生成器的值，因为生成第二个项目很可能依赖于已生成的第一个项目。
回答by Kathy Van Stone
You have to walk through the generator anyway to get the first item so you might as well make a list. Try
无论如何，您必须遍历生成器才能获得第一个项目，因此您不妨列个清单。尝试
reversed(list(g))
where gis a generator.
哪里g是发电机。
reversed(tuple(g))
would work as well (I didn't check to see if there is a significant difference in performance).
也会起作用（我没有检查性能是否有显着差异）。
