Question

这是怎么回事？...

所以，这是我的情况。我正在准备一个Web剪贴簿，以便在此脚本的某个点上，我决定使用装饰器来处理一些URL。该装饰器具有一个参数（URL），该参数应在for循环语句的帮助下动态更改，如示例脚本所示：

from functools import wraps
import logging

logging.basicConfig(level=logging.INFO)

def cycle(url):
    def outer_wrapper(func):
        state = 0
        @wraps(func)
        def inner_wrapper(**kwargs):
            nonlocal state
            state += 1
            kwargs['url'] = url
            if state == 1:
                logging.info('Returning result at first execution on {} with: '
                             'state => {}, kwargs => {}'.format(func, state, kwargs))
                return func(**kwargs)
            else:
                logging.info('Returning result at upcoming executions on {} with: '
                             'state => {}, kwargs => {}'.format(func, state, kwargs))
                return func(**kwargs)

        return inner_wrapper
    return outer_wrapper


def print_url(url):
    print('Returned from print_url function:', url)

links = ['an-url', 'another-url']

for link in links:
    # Decorator
    print_url = cycle(link)(print_url)
    print_url()

但是，输出显示出意外的结果，至少对我而言。

INFO:root:Returning result at first execution on <function print_url at 0x000002202FD68D08> with: state => 1, kwargs => {'url': 'an-url'}
Returned from print_url function: an-url
INFO:root:Returning result at first execution on <function print_url at 0x000002202FE196A8> with: state => 1, kwargs => {'url': 'another-url'}
INFO:root:Returning result at upcoming executions on <function print_url at 0x000002202FD68D08> with: state => 2, kwargs => {'url': 'an-url'}
Returned from print_url function: an-url

装饰器保留第一次调用print_url()函数的地址和参数。我已经阅读了很多有关垃圾回收，弱引用和标准库的functools的文章，但是我无法弄清楚如何“重新启动”此装饰器，以使其在迭代过程中收到新的参数。 / p>

如果可能的话，有人会提示如何解决此问题吗？

Answer 1

之所以发生这种情况，是因为您正在重新装饰已经装饰的功能。

第一次迭代后：

for link in links:
    # Decorator
    print_url = cycle(link)(print_url)
    print_url()

然后print_url指向inner_wrapper。您再次装饰inner_wrapper。这与垃圾回收无关，这只是您编写此操作所要做的。

如果您删除wraps，这一点将变得更加清楚：

def cycle(url):
    def outer_wrapper(func):
        state = 0
        def inner_wrapper(**kwargs):
            nonlocal state
            state += 1
            kwargs['url'] = url
            if state == 1:
                print('Returning result at first execution on {} with: '
                             'state => {}, kwargs => {}'.format(func, state, kwargs))
                return func(**kwargs)
            else:
                print('Returning result at upcoming executions on {} with: '
                             'state => {}, kwargs => {}'.format(func, state, kwargs))
                return func(**kwargs)

        return inner_wrapper
    return outer_wrapper


def print_url(url):
    print('Returned from print_url function:', url)

links = ['an-url', 'another-url']

for i, link in enumerate(links):
    print("Iteration :", i)
    print_url = cycle(link)(print_url)
    print_url()

终端输出：

Iteration : 0
Returning result at first execution on <function print_url at 0x1060892f0> with: state => 1, kwargs => {'url': 'an-url'}
Returned from print_url function: an-url
Iteration : 1
Returning result at first execution on <function cycle.<locals>.outer_wrapper.<locals>.inner_wrapper at 0x106089378> with: state => 1, kwargs => {'url': 'another-url'}
Returning result at upcoming executions on <function print_url at 0x1060892f0> with: state => 2, kwargs => {'url': 'an-url'}
Returned from print_url function: an-url

如果要访问原始函数，建议不要在循环外保留对它的引用，不要将装饰器的结果分配给该变量，例如：

original_function = print_url
for link in links:
    # Decorator
    print_url = cycle(link)(original_function)
    print_url()
    print_url = original_function

只是为了好玩，您可以在每次迭代中恢复它，从而达到以下效果：

for link in links:
    # Decorator
    print_url = cycle(link)(print_url)
    print_url()
    closure = print_url.__closure__
    idx_func = print_url.__code__.co_freevars.index('func')
    print_url = closure[idx_func].cell_contents

但是...那只是一个可怕的烂摊子，暴露出一堆内部细节，最好留在引擎盖下。

从根本上说，我不确定为什么这里需要装饰器，即使用该装饰器有什么好处？

有没有一种方法可以“解决”装饰器的保留状态？

1 个答案: