for 循环如何评估其参数

Question

我的问题很简单。

for 循环是否每次都会评估它使用的参数？

如：

for i in range(300):

python 是否为这个循环的每次迭代创建一个包含 300 个项目的列表？

如果是，这是避免它的方法吗？

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)):

是每次都应用反向过程，还是每次迭代都计算长度？ （我对 python2 和 python3 都要求这个）

如果不是，Python 在迭代时如何评估可迭代对象的更改？

Answer 1

创建什么对象取决于您循环返回的 Iterable 的 __iter__ 方法。

当迭代一个本身不是迭代器的迭代器时，Python 通常会创建一个迭代器。在 Python2 中，range 返回一个列表，它是一个 Iterable，并且有一个 __iter__ 方法返回一个 Iterator。

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

for in sequence: do something 语法基本上是执行此操作的简写：

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

这是一个演示，其中包含一些打印语句，以阐明使用 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，__iter__ 只被调用一次，因此只创建了一个 Iterator 对象。

Answer 2

Range 在这种情况下会创建一个包含 300 个整数的数组。它不会创建 300 个整数的数组 300 次。它不是每一个有效的。如果您使用 xrange ，它将创建一个不会占用几乎一样多内存的可迭代对象。 https://docs.python.org/2/library/functions.html#xrange

example.py

​​>

for i in xrange(300): #low memory foot print, similar to a normal loop
    print(i)

Answer 3

不用担心，迭代器只会被评估一次。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，它不是相当等效的，因为break 的工作方式会有所不同。请记住，您可以将 else 添加到 for 循环中，但这在上面的代码中不起作用。