python – 在迭代迭代器时重新分配迭代器的值

2019年8月4日 190次阅读

First of all a disclaimer:

I don’t want to use a code like this, I am aware it is a bad practice. As well I am not interested in tips on how to improve it, to make it right. What interests me is a theory.

为什么这样的代码在python 3.6中有效：

ls = range(5)
for elem_a in ls:

    ls = range(5, 10)
    for elem_b in ls:
        print(elem_a, elem_b)

我在迭代它时重新分配ls的值.在ls中第一次执行elem_a时,第一次迭代中的ls值是否存储在内存中？

最佳答案 重新分配您正在循环的变量没有任何效果,因为不会为每次迭代重新评估变量.实际上,循环内部循环遍历
iterator,而不是覆盖范围对象.

基本上如果你有这样的循环：

seq = range(5)
for elem in seq:
    seq = something_else

Python将其重写为以下内容：

seq = range(5)

loop_iter = iter(seq)  # obtain an iterator
while True:
    try:
        elem = next(loop_iter)  # get the next element from the iterator
    except StopIteration:
        break  # the iterator is exhausted, end the loop

    # execute the loop body
    seq = something_else

这个的关键方面是循环有自己对iter(seq)的引用存储在loop_iter中,所以自然地重新分配seq对循环没有影响.

所有这些都在compound statement documentation中解释：

06002

The expression list is evaluated once; it should yield an iterable
object. An iterator is created for the result of the expression_list.
The suite is then executed once for each item provided by the
iterator, in the order returned by the iterator.

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