需要在python中的迭代器的开头添加一个元素 [英] Need to add an element at the start of an iterator in python

问题描述

我有一个程序如下：

a=reader.next()
if *some condition holds*:
    #Do some processing and continue the iteration
else:
    #Append the variable a back to the iterator
    #That is nullify the operation *a=reader.next()*

如何在迭代器的开头添加元素？
（或者有更简单的方法吗？）

How do I add an element to the start of the iterator? (Or is there an easier way to do this?)

编辑：好的，让我这样说吧。我需要迭代器中的下一个元素而不删除它。
我该怎么做>？

OK let me put it this way. I need the next element in an iterator without removing it. How do I do this>?

推荐答案

Python迭代器本身功能非常有限 - 没有附加或类似的东西。您需要将通用迭代器包装在添加该功能的包装器中。例如：

Python iterators, as such, have very limited functionality -- no "appending" or anything like that. You'll need to wrap the generic iterator in a wrapper adding that functionality. E.g.:

class Wrapper(object):
  def __init__(self, it):
    self.it = it
    self.pushedback = []
  def __iter__(self):
    return self
  def next(self):
    if self.pushedback:
      return self.pushedback.pop()
    else:
      return self.it.next()
  def pushback(self, val):
    self.pushedback.append(val)

这是Python 2.5（也应该在2.6中工作） - 建议2.6和必须为3.any（使用 next（self.it）而不是 self.it.next（）并定义 __ next __ 而不是 next ）。

This is Python 2.5 (should work in 2.6 too) -- slight variants advised for 2.6 and mandatory for 3.any (use next(self.it) instead of self.it.next() and define __next__ instead of next).

编辑：OP现在说他们需要的是在没有消费的情况下向前看。包装仍然是最佳选择，但另一种选择是：

Edit: the OP now says what they need is "peek ahead without consuming". Wrapping is still the best option, but an alternative is:

import itertools
   ...
o, peek = itertools.tee(o)
if isneat(peek.next()): ...

这不会提前 o （如果您决定要这样做，请记得提前; - ）。

this doesn't advance o (remember to advance it if and when you decide you DO want to;-).

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