让python迭代器倒退？ [英] Making a python iterator go backwards?

问题描述

有没有让python列表迭代器倒退？

Is there anyway to make a python list iterator to go backwards?

基本上我有这个

class IterTest(object):
    def __init__(self, data):
        self.data = data
        self.__iter = None

    def all(self):
        self.__iter = iter(self.data)
        for each in self.__iter:
            mtd = getattr(self, type(each).__name__)
            mtd(each)

    def str(self, item):
        print item

        next = self.__iter.next()
        while isinstance(next, int):
            print next
            next = self.__iter.next()

    def int(self, item):
        print "Crap i skipped C"

if __name__ == '__main__':
    test = IterTest(['a', 1, 2,3,'c', 17])
    test.all()

运行此代码会产生输出：

Running this code results in the output:

a
1
2
3
Crap i skipped C

我知道为什么它给了我输出，但有一种方法我可以倒退str（）方法，一步？

I know why it gives me the output, however is there a way i can step backwards in the str() method, by one step?

编辑

好吧也许使这更清楚。我不想做完全反向，基本上我想知道是否有一种简单的方法在python中做相当于双向迭代器？

Okay maybe to make this more clear. I don't want to do a full reverse, basically what i want to know if there is an easy way to do the equivalent of a bidirectional iterator in python?

推荐答案

不，通常你不能让Python迭代器倒退。但是，如果您只想退一步，可以尝试这样的事情：

No, in general you cannot make a Python iterator go backwards. However, if you only want to step back once, you can try something like this:

def str(self, item):
    print item

    prev, current = None, self.__iter.next()
    while isinstance(current, int):
        print current
        prev, current = current, self.__iter.next()

然后您可以随时访问上一个元素在上一步。

You can then access the previous element any time in prev.

如果你真的需要一个双向迭代器，你可以自己实现一个，但它可能会介绍比上面的解决方案更开销：

If you really need a bidirectional iterator, you can implement one yourself, but it's likely to introduce even more overhead than the solution above:

class bidirectional_iterator(object):
    def __init__(self, collection):
        self.collection = collection
        self.index = 0

    def next(self):
        try:
            result = self.collection[self.index]
            self.index += 1
        except IndexError:
            raise StopIteration
        return result

    def prev(self):
        self.index -= 1
        if self.index < 0:
            raise StopIteration
        return self.collection[self.index]

    def __iter__(self):
        return self

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