如何从 __next__ 方法产生？答案

【问题标题】：How to yield from __next__ method?如何从 __next__ 方法产生？
【发布时间】：2019-05-09 07:10:09
【问题描述】：

我认为下面的代码会表达想要的结果：

class LockIterator(object):

    def __init__(self, lock_list):
        self.lock_list = lock_list

    def __iter__(self):
        return self

    def __next__(self):

        for resource in self.lock_list:
            print( "Locking N resources" )

            yield resource
            print( "Unlocking N resources" )

        print( "Unlocking remaining resources" )
        raise StopIteration


for item in LockIterator( ["Resource 1", "Resource 2", "Resource 3"] ):
    print("Safe resource usage:", item)

但是，在 Python 上运行它，我得到了一个无限循环：

Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BDA24938>
Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BB8AEE60>
Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BDA24938>
Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BB8AEE60>
Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BDA24938>
Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BB8AEE60>
Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BDA24938>
Safe resource usage: <generator object LockIterator.__next__ at 0x000001A8BB8AEE60>
...

在我的想象中，它应该是这样运行的：

Locking N resources
Safe resource usage: Resource 1
Unlocking N resources
Locking N resources
Safe resource usage: Resource 2
Unlocking N resources
Locking N resources
Safe resource usage: Resource 3
Unlocking N resources
Unlocking remaining resources

您知道如何在普通的 for 循环中自动强制执行此行为吗？

for item in LockIterator( ["Resource 1", "Resource 2", "Resource 3"] ):
    print("Safe resource usage:", item)

【问题讨论】：

标签： python-3.x for-loop iterator generator yield

【解决方案1】：

__next__ 在每次 for 循环需要一个新项目时被调用。因为你的__next__ 是一个生成器，所以每次都会返回。

相反，你可以去掉这个类，只写一个生成器：

def LockIterator(lock_list):
    # better name this lockify or something else in snake_case
    for resource in lock_list:
        print("Locking N resources")

        yield resource
        print("Unlocking N resources")

    print("Unlocking remaining resources")

【讨论】：

【解决方案2】：

我还设法通过删除 __next__ 并将其主体移动到 __iter__ 方法来修复它：

class LockIterator(object):

    def __init__(self, lock_list):
        self.lock_list = lock_list

    def __iter__(self):

        for resource in self.lock_list:
            print( "Locking N resources" )

            yield resource
            print( "Unlocking N resources" )

        print( "Unlocking remaining resources" )

for item in LockIterator( ["Resource 1", "Resource 2", "Resource 3"] ):
    print("Safe resource usage:", item)

【讨论】：