发电机很奇怪(The generator works strange)

发电机很奇怪(The generator works strange) a = [1, 2, 3] b = (c for c in a if c in a) a = [2, 3, 4] print(list(b)) def d(expr): for c in expr: if c in expr: yield c a1 = [1,2,3] t = d(a1) a1 = [2,3,4] print(list(t))

输出：

[2, 3] [1, 2, 3]

问题：1）在第一个变体中，循环保留列表的旧值（[1,2,3]），但条件表达式采用新的列表a（[2,3,4]）。 2）我自己的生成器如何给出除生成表达之外的其他结果？

例如，我替换了第一个例子的真实列表，我希望这将更清楚我的第一个问题。

a = [1, 2, 3] b = (c for c in [1,2,3] if c in [2, 3, 4]) a = [2, 3, 4] print(list(b))

输出：

[2,3]a = [1, 2, 3] b = (c for c in a if c in a) a = [2, 3, 4] print(list(b)) def d(expr): for c in expr: if c in expr: yield c a1 = [1,2,3] t = d(a1) a1 = [2,3,4] print(list(t))

output:

[2, 3] [1, 2, 3]

question: 1) in the first variant, the cycle retains the old value of the list ([1,2,3]), but the conditional expression takes a new list a([2,3,4]). 2) how my own generator give me other result than generative expression?

For example, I replaced a on the real lists of the first example, I hope this will be more clear my first question.

a = [1, 2, 3] b = (c for c in [1,2,3] if c in [2, 3, 4]) a = [2, 3, 4] print(list(b))

Output:

[2,3]

最满意答案

根据PEP 289 ，我们可以按如下方式重写您的生成器表达式：

# b = (c for c in a if c in a) def __gen(expr): for c in expr: if c in a: yield c b = __gen(iter(a))

这里，立即捕获用于for循环的[1, 2, 3]的引用（当你创建b ），但是检查中的if c in a使用等于[2, 3, 4]的新全局a [2, 3, 4] 。

另一方面，当您编写生成器函数时， a1传递给函数并存储以供稍后在for和if x in y表达式。 分配a1 = [2, 3, 4]无效。

要使函数与生成器表达式执行相同的操作，您需要做的就是将if c in expr替换if c in a1 。

As per PEP 289, we can rewrite your generator expression as follows:

# b = (c for c in a if c in a) def __gen(expr): for c in expr: if c in a: yield c b = __gen(iter(a))

Here, a reference to [1, 2, 3] for use in the for loop is captured immediately (when you create b), but the if c in a check uses the new global a which is equal to [2, 3, 4].

On the other hand, when you write a generator function, a1 is passed to the function and stored for later use in both for and if x in y expressions. Assigning a1 = [2, 3, 4] has no effect.

To make the function do the same thing as the generator expression, all you need to do is replace if c in expr with if c in a1.