問題描述

Python 中的其他空對象評估為 False —— 我怎樣才能讓迭代器/生成器也這樣做?

Other empty objects in Python evaluate as False -- how can I get iterators/generators to do so as well?

推薦答案

默認(rèn)情況下，Python 中的所有對象都評估為 True.為了支持 False 評估，對象的類必須具有 __len__ 方法(0 -> False)，或 __nonzero__ 方法 (False -> False).注意:__nonzero__ ==> __bool__ 在 Python 3.x 中.

By default all objects in Python evaluate as True. In order to support False evaluations the object's class must have either a __len__ method (0 -> False), or a __nonzero__ method (False -> False). Note: __nonzero__ ==> __bool__ in Python 3.x.

因為迭代器協(xié)議有意保持簡單，并且因為有許多類型的迭代器/生成器在嘗試生成它們之前無法知道是否還有更多值要生成，所以 True/False 評估不是迭代器協(xié)議的一部分.

Because the iterator protocol is intentionally kept simple, and because there are many types of iterators/generators that aren't able to know if there are more values to produce before attempting to produce them, True/False evaluation is not part of the iterator protocol.

如果你真的想要這種行為，你必須自己提供.一種方法是將生成器/迭代器包裝在提供缺失功能的類中.

If you really want this behavior, you have to provide it yourself. One way is to wrap the generator/iterator in a class that provides the missing functionality.

請注意，此代碼僅在引發(fā) StopIteration 后評估為 False .

Note that this code only evaluates to False after StopIteration has been raised.

作為獎勵，此代碼適用于 pythons 2.4+

As a bonus, this code works for pythons 2.4+

try:
    next
except NameError:       # doesn't show up until python 2.6
    def next(iter):
        return iter.next()

Empty = object()

class Boolean_Iterator(object):
    """Adds the abilities
    True/False tests:  True means there /may/ be items still remaining to be used
    """
    def __init__(self, iterator):
        self._iter = iter(iterator)
        self._alive = True
    def __iter__(self):
        return self
    def __next__(self):
        try:
            result = next(self._iter)
        except StopIteration:
            self._alive = False
            raise
        return result
    next = __next__                     # python 2.x
    def __bool__(self):
        return self._alive
    __nonzero__ = __bool__              # python 2.x

如果您還想要前瞻(或窺視)行為，則此代碼可以解決問題(它的計算結(jié)果為 False before StopIteration被提出):

If you also want look-ahead (or peek) behavior, this code will do the trick (it evaluates to False before StopIteration is raised):

try:
    next
except NameError:       # doesn't show up until python 2.6
    def next(iter):
        return iter.next()

Empty = object()

class Iterator(object):
    """Adds the abilities
    True/False tests:  True means there are items still remaining to be used
    peek(): get the next item without removing it from the sequence
    """
    def __init__(self, iterator):
        self._iter = iter(iterator)
        self._peek = Empty
        self.peek()
    def __next__(self):
        peek, self._peek = self._peek, Empty
        self.peek()
        if peek is not Empty:
            return peek
        raise StopIteration
    next = __next__                     # python 2.x
    def __bool__(self):
        return self._peek is not Empty
    __nonzero__ = __bool__              # python 2.x
    def peek(self):
        if self._peek is not Empty:
            return self._peek
        self._peek = next(self._iter, Empty)
        return self._peek

請記住，當(dāng)?shù)讓拥?生成器的時間與其產(chǎn)生的值相關(guān)時，窺視行為是不合適的.

Keep in mind that peek behaviour is not appropriate when the timing of the underlying iterator/generator is relevant to its produced values.

另外請記住，第三方代碼，可能還有標(biāo)準(zhǔn)庫，可能依賴于迭代器/生成器，總是評估為 True.如果你想在沒有 bool 的情況下查看，請刪除 __nonzero__ 和 __bool__ 方法.

Also keep in mind that third-party code, and possibly the stdlib, may rely on iterators/generators always evaluating to True. If you want peek without bool, remove the __nonzero__ and __bool__ methods.

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