Python，为什么i = + 1不会导致无限循环？

Python，为什么i = + 1不会导致无限循环？

Python for循环是基于迭代器的“ for-each”循环。每次迭代开始时都会重新分配迭代变量。换句话说，以下循环：

In [15]: nums = 1,2,5,8In [16]: for num in nums:    ...:     print(num)    ...:1258

等效于：

In [17]: it = iter(nums)    ...: while True:    ...:     try:    ...:         num = next(it)    ...:     except StopIteration:    ...:         break    ...:     print(num)    ...:1258

同样，以下循环是等效的：

In [19]: for num in nums:    ...:     print("num:", num)    ...:     num += 1    ...:     print("num + 1:", num)    ...:    ...:num: 1num + 1: 2num: 2num + 1: 3num: 5num + 1: 6num: 8num + 1: 9In [20]: it = iter(nums)    ...: while True:    ...:     try:    ...:         num = next(it)    ...:     except StopIteration:    ...:         break    ...:     print("num:", num)    ...:     num += 1    ...:     print("num + 1:", num)    ...:num: 1num + 1: 2num: 2num + 1: 3num: 5num + 1: 6num: 8num + 1: 9

请注意，C样式的for循环在Python中不存在，但是您始终可以编写while循环（c样式的for循环本质上是while循环的语法糖）：

for(int i = 0; i < n; i++){    // do stuff}

等效于：

i = 0while i < n:    # do stuff    i += 1

请注意，不同之处在于，在这种情况下，迭代 取决于

i

# do stuff

，修改的任何内容

i

都会影响迭代，而在前一种情况下，迭代取决于
迭代器 。注意，如果我们确实修改了迭代器，则迭代会受到影响：

In [25]: it = iter(nums) # give us an iterator    ...: for num in it:    ...:     print(num)    ...:     junk = next(it) # modifying the iterator by taking next value    ...:    ...:15