Python for循环如何计算其参数

我的问题很简单

for

循环是否每次都计算它使用的参数

例如：

for i in range(300):

python是否为该循环的每个迭代创建了一个包含300项的列表

如果是，这是一种避免方法吗

lst = range(300)
for i in lst:
    #loop body

类似这样的代码示例也是如此

for i in reversed(lst):

for k in range(len(lst)):

是每次应用反向过程，还是每次迭代计算长度？（我对蟒蛇2和蟒蛇3都这样要求）

---

如果不是，Python在迭代iterable时如何计算iterable上的更改？

不用担心，迭代器将只计算一次。它最终大致相当于这样的代码：

it = iter(range(300))
while True:
    try:
        i = next(it)
    except StopIteration:
        break
    ... body of loop ...

---

请注意，它不是完全等效的，因为

break

的工作方式不同。请记住，您可以将

else

添加到

for

循环中，但这在上述代码中不起作用。

在这种情况下，Range会创建一个300整数的数组。它不会创建300整数的数组300次。它不是很有效率。如果您使用xrange，它将创建一个不会占用太多内存的iterable对象

示例.py

---

创建什么对象取决于在返回上循环的Iterable的

\uuuuu iter\uuuu

方法

通常，Python在迭代本身不是迭代器的Iterable时会创建一个迭代器。在Python2中，

range

返回一个列表，该列表是一个Iterable，具有一个返回迭代器的

\uuuuuuuuuuu

方法

>>> from collections import Iterable, Iterator
>>> isinstance(range(300), Iterable)
True
>>> isinstance(range(300), Iterator)
False
>>> isinstance(iter(range(300)), Iterator)
True

in-sequence:do something的

语法基本上是这样做的简写：

it = iter(some_iterable) # get Iterator from Iterable, if some_iterable is already an Iterator, __iter__ returns self by convention
while True:
    try:
        next_item = next(it)
        # do something with the item
    except StopIteration:
        break

下面是一个演示，其中包含一些打印语句，以澄清使用for循环时发生的情况：

class CapitalIterable(object):
    'when iterated over, yields capitalized words of string initialized with'
    def __init__(self, stri):
        self.stri = stri

    def __iter__(self):
        print('__iter__ has been called')
        return CapitalIterator(self.stri)

        # instead of returning a custom CapitalIterator, we could also
        # return iter(self.stri.title().split())
        # because the built in list has an __iter__ method

class CapitalIterator(object):
    def __init__(self, stri):
        self.items = stri.title().split()
        self.index = 0

    def next(self): # python3: __next__
        print('__next__ has been called')
        try:
            item = self.items[self.index]
            self.index += 1
            return item
        except IndexError:
            raise StopIteration

    def __iter__(self):
        return self

c = CapitalIterable('The quick brown fox jumps over the lazy dog.')
for x in c:
    print(x)

输出：

__iter__ has been called
__next__ has been called
The
__next__ has been called
Quick
__next__ has been called
Brown
__next__ has been called
Fox
__next__ has been called
Jumps
__next__ has been called
Over
__next__ has been called
The
__next__ has been called
Lazy
__next__ has been called
Dog.
__next__ has been called

如您所见，
\uu iter\uu
只被调用一次，因此只创建了一个迭代器对象。
或者在Python2或Python3中，
i=next（it）
。此外，尽管在整个循环中使用的是同一个迭代器，但它所迭代的对象可能会发生变化，这就是为什么不建议这样做的原因，例如，在迭代列表元素时修改列表。谢谢，我刚刚找到了一种方法来检查我所问的问题。我可以定义一个返回iterable（不是生成器）的函数，在其中添加一个print语句，然后检查它将打印多少次。如果您对优化感兴趣，我会看看“将代码转换为漂亮的、惯用的Python”-很多很棒的Python代码。您已经得到了答案，但值得一提的是这个范围（）仅返回Python 2中的列表。在Python3中，
range（300）
创建一个range对象，它是一个具有内部状态的生成器。这意味着您可以在不耗尽内存的情况下执行诸如
range（10000000000000000）
之类的操作。在Python2中，可以使用
xrange
获得相同的效果。
__iter__ has been called
__next__ has been called
The
__next__ has been called
Quick
__next__ has been called
Brown
__next__ has been called
Fox
__next__ has been called
Jumps
__next__ has been called
Over
__next__ has been called
The
__next__ has been called
Lazy
__next__ has been called
Dog.
__next__ has been called