python - Python for loop implementation

Question

Can someone tell me how exactly Python's for loops are implemented? The reason I'm asking this is because I'm getting different behavior in the following two for loops when I expect the same behavior (assuming cases is just a set of elements):

First for loop:

for case in cases:
    blah

Second for loop:

for i in range(len(cases)):
    case = cases[i]
    blah

I'm running my code in a multi-threaded environment.

Basically, I'm wondering whether Python's for loop's iterating over a set (as in the first for loop) is simply a quickhand way of the second one. What exactly happens when we use the python for loop, and is there any underlying optimization/ implementation that may be causing the behavior difference I'm observing?

Answer 1

不，第二种格式完全不同。

for循环调用iter()to-loop-over 序列，并使用next()对结果的调用。认为它相当于：

iterable = iter(cases):
while True:
    try:
        case = next(iterable)
    except StopIteration:
        break

    # blah

调用列表的结果iter()是一个列表迭代器对象：

>>> iter([])
<list_iterator object at 0x10fcc6a90>

该对象保留对原始列表的引用并跟踪它所在的索引。该索引从 0 开始并递增，直到列表被完全迭代。

不同的对象可以返回具有不同行为的不同迭代器。混入线程后，您最终可能会cases用其他东西替换，但迭代器仍会引用旧序列。

Answer 2

l = [1, 2, 3, 4, 5]
l = iter(l)
while True:
    try:
        print l.next()
    except StopIteration:
        exit()

Answer 3

我没有得到任何区别，请在下面查看，您是否正在尝试..

>>> cases = [1,2,3]
>>> for case in cases:
...     print case
...
1
2
3
>>> i=0
>>> for i in range(len(cases)):
...     print cases[i]
...
1
2
3
>>>