生成器表达式使用生成器创建后分配的列表 [英] Generator expression uses list assigned after the generator's creation

问题描述

我找到了这个例子，但我不明白为什么它无法正常工作? 我以为它必须输出[1, 8, 15]或[2, 8, 22].

I found this example and I can't understand why it works unpredictably? I supposed it must output [1, 8, 15] or [2, 8, 22].

array = [1, 8, 15]
g = (x for x in array if array.count(x) > 0)
array = [2, 8, 22]
print(list(g))


>>>[8]

推荐答案

原因是，在创建时，生成器(a for b in c if d)仅评估c(有时有时使b以及可预测的).但是a，b，d是在消耗时间(每次迭代)进行评估的.在这里，当评估d(array.count(x) > 0)时，它将使用封闭范围中的array的 current 绑定.

The reason is that, at creation time, the generator (a for b in c if d) only evaluates c (which sometimes makes b predictable as well). But a, b, d are evaluated at consumption time (at each iteration). Here, it uses the current binding of array from the enclosing scope when evaluating d (array.count(x) > 0).

例如，您可以这样做:

g = (x for x in [] if a)

没有事先声明a.但是，必须确保在消耗生成器时a存在.

Without having declared a in advance. But, you have to make sure a exists when the generator is consumed.

但是您不能做类似的事情:

But you cannot do similarly:

g = (x for x in a if True)

根据要求:

您可以使用常见的生成器功能观察相似(但不完全相同)的模式:

You can observe similar (however not identical) patterns with a common generator function:

def yielder():
    for x in array:
        if array.count(x) > 0:
            yield x

array = [1, 8, 15]
y = yielder()
array = [2, 8, 22]
list(y)
# [2, 8, 22]

生成器函数在消耗之前不会执行其任何主体.因此，即使for循环标头中的array也绑定得较晚.甚至还有一个更令人不安的示例，其中我们在迭代过程中退出" array:

The generator function does not execute any of its body ahead of consumption. Hence, even the array in the for-loop header is bound late. An even more disturbing example occurs where we "switch out" array during iteration:

array = [1, 8, 15]
y = yielder()
next(y)
# 1
array = [3, 7]
next(y)  # still iterating [1, 8, 15], but evaluating condition on [3, 7]
# StopIteration raised

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