Python：'for ... in'语句评估行为(Python: 'for … in' statement evaluation behavior)

Python：'for ... in'语句评估行为(Python: 'for … in' statement evaluation behavior)

我在Python中有以下代码示例：

def get_objects(): print 'Called get_objects' return [1, 2, 3, 4, 5] def main(): for i in get_objects(): print i main()

输出如下：

Called get_objects 1 2 3 4 5

所以我的问题是为什么get_objects（）只调用一次？ 为什么每次迭代都没有获取数据？

I have following code sample in python:

def get_objects(): print 'Called get_objects' return [1, 2, 3, 4, 5] def main(): for i in get_objects(): print i main()

Output for this as following:

Called get_objects 1 2 3 4 5

So my question is why get_objects() called only once? Why it is not fetched data every time of itteration?

最满意答案

因为这正是Python for循环的工作原理。 in运算符右侧的表达式（在评估时必须返回一个可迭代表达式）仅在循环开始之前计算一次 。 然后，for循环遍历该评估返回的迭代器。

以下是文档 *的摘录：

for语句用于迭代序列的元素（如字符串，元组或列表）或其他可迭代对象：

for_stmt ::= "for" target_list "in" expression_list ":" suite ["else" ":" suite]

表达式列表被评估一次 ; 它应该产生一个可迭代的对象。

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*注意：我添加了粗体。

Because that is exactly how for-loops work in Python. The expression on the right of the in operator (which must return an iterable when evaluated) is evaluated only once, before looping commences. Then, the for-loop loops over the iterable returned by that evaluation.

Below is an excerpt from the docs*:

The for statement is used to iterate over the elements of a sequence (such as a string, tuple or list) or other iterable object:

for_stmt ::= "for" target_list "in" expression_list ":" suite ["else" ":" suite]

The expression list is evaluated once; it should yield an iterable object.

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*Note: I added the bold.