問題描述

我有以下字典包裝器:

class MyDict:
    def __init__(self):
        self.container = {}

    def __setitem__(self, key, value):
        self.container[key] = value

    def __getitem__(self, key):
        return self.container[key]

    def __iter__(self):
        return self

    def next(self):
        pass

dic = MyDict()
dic['a'] = 1
dic['b'] = 2

for key in dic:
    print key

我的問題是我不知道如何實現 next 方法以使 MyDict 可迭代.任何意見，將不勝感激.

My problem is that I don't know how to implement the next method to make MyDict iterable. Any advice would be appreciated.

推薦答案

字典本身不是迭代器(只能迭代一次).您通常將它們設為 iterable，您可以為其生成多個 iterators 的對象.

Dictionaries are themselves not an iterator (which can only be iterated over once). You usually make them an iterable, an object for which you can produce multiple iterators instead.

完全刪除 next 方法，并讓 __iter__ 在每次調用時返回一個可迭代對象.這可以像返回 self.container 的迭代器一樣簡單:

Drop the next method altogether, and have __iter__ return an iterable object each time it is called. That can be as simple as just returning an iterator for self.container:

def __iter__(self):
    return iter(self.container)

如果你必須讓你的類成為一個迭代器，你必須以某種方式跟蹤當前的迭代位置并在到達結束"時提高 StopIteration.一個簡單的實現可能是在第一次調用 __iter__ 時將 iter(self.container) 對象存儲在 self 上:

If you must make your class an iterator, you'll have to somehow track a current iteration position and raise StopIteration once you reach the 'end'. A naive implementation could be to store the iter(self.container) object on self the first time __iter__ is called:

def __iter__(self):
    return self

def next(self):
    if not hasattr(self, '_iter'):
        self._iter = iter(self.container)
    return next(self._iter)

此時 iter(self.container) 對象會為您跟蹤迭代位置，并在到達結束時引發 StopIteration.如果底層字典被更改(添加或刪除鍵)并且迭代順序被破壞，它也會引發異常.

at which point the iter(self.container) object takes care of tracking iteration position for you, and will raise StopIteration when the end is reached. It'll also raise an exception if the underlying dictionary was altered (had keys added or deleted) and iteration order has been broken.

另一種方法是每次只存儲整數位置并索引到 list(self.container) 中，而忽略插入或刪除可以改變迭代順序的事實字典:

Another way to do this would be to just store in integer position and index into list(self.container) each time, and simply ignore the fact that insertion or deletion can alter the iteration order of a dictionary:

_iter_index = 0

def __iter__(self):
    return self

def next(self):
    idx = self._iter_index
    if idx is None or idx >= len(self.container):
        # once we reach the end, all iteration is done, end of.
        self._iter_index = None
        raise StopIteration()
    value = list(self.container)[idx]
    self._iter_index = idx + 1
    return value

在這兩種情況下，您的對象都是一個迭代器，只能迭代一次.一旦到達終點，就不能再重新開始了.

In both cases your object is then an iterator that can only be iterated over once. Once you reach the end, you can't restart it again.

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