处理上下文管理器中的异常

Question

我有一些代码试图访问资源，但有时它是不可用的，并导致异常。我试图使用上下文管理器实现重试引擎，但是我无法处理上下文管理器的__enter__上下文中的调用方引发的异常。

class retry(object):
    def __init__(self, retries=0):
        self.retries = retries
        self.attempts = 0
    def __enter__(self):
        for _ in range(self.retries):
            try:
                self.attempts += 1
                return self
            except Exception as e:
                err = e
    def __exit__(self, exc_type, exc_val, traceback):
        print 'Attempts', self.attempts

以下是一些只会引发异常的例子(我希望能处理这些异常)

>>> with retry(retries=3):
...     print ok
... 
Attempts 1
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
NameError: name 'ok' is not defined
>>> 
>>> with retry(retries=3):
...     open('/file')
... 
Attempts 1
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
IOError: [Errno 2] No such file or directory: '/file'

有没有办法拦截这些异常并在上下文管理器中处理它们？

Answer 1

引用__exit__的话，

如果提供了一个异常，并且该方法希望抑制异常(即阻止它被传播)，那么它应该返回一个真值。否则，异常将在退出此方法时正常处理。

默认情况下，如果不显式地从函数返回值，Python将返回None，这是一个falsy值。在您的示例中，__exit__返回None，这就是为什么允许exeception通过__exit__。

所以，返回一个真实的值，如下所示

class retry(object):

    def __init__(self, retries=0):
        ...


    def __enter__(self):
        ...

    def __exit__(self, exc_type, exc_val, traceback):
        print 'Attempts', self.attempts
        print exc_type, exc_val
        return True                                   # or any truthy value

with retry(retries=3):
    print ok

输出将是

Attempts 1
<type 'exceptions.NameError'> name 'ok' is not defined

如果您想拥有重试功能，可以使用装饰器实现它，如下所示

def retry(retries=3):
    left = {'retries': retries}

    def decorator(f):
        def inner(*args, **kwargs):
            while left['retries']:
                try:
                    return f(*args, **kwargs)
                except NameError as e:
                    print e
                    left['retries'] -= 1
                    print "Retries Left", left['retries']
            raise Exception("Retried {} times".format(retries))
        return inner
    return decorator


@retry(retries=3)
def func():
    print ok

func()

Answer 2

要处理__enter__方法中的异常，最简单(也不那么令人惊讶)的事情是将with语句本身包装在一个try-除了子句中，然后简单地引发异常-

但是，with块的设计显然不能像这样工作--仅凭自己的“可检索性”--这里存在一些误解：

def __enter__(self):
    for _ in range(self.retries):
        try:
            self.attempts += 1
            return self
        except Exception as e:
            err = e

返回self后，__enter__运行的上下文就不再存在--如果在with块中发生错误，它将自然地流向__exit__方法。不，__exit__方法无论如何不能使执行流返回到with块的开头。

你可能更想要这样的东西：

class Retrier(object):

    max_retries = 3

    def __init__(self, ...):
         self.retries = 0
         self.acomplished = False

    def __enter__(self):
         return self

    def __exit__(self, exc, value, traceback):
         if not exc:
             self.acomplished = True
             return True
         self.retries += 1
         if self.retries >= self.max_retries:
             return False
         return True

....

x = Retrier()
while not x.acomplished:
    with x:
        ...

Answer 3

我认为这件事很容易，其他人似乎想得太多了。只需将资源获取代码放在__enter__中，然后尝试返回，不是self，而是获取的资源。代码：

def __init__(self, retries):
    ...
    # for demo, let's add a list to store the exceptions caught as well
    self.errors = []

def __enter__(self):
    for _ in range(self.retries):
        try:
            return resource  # replace this with real code
        except Exception as e:
            self.attempts += 1
            self.errors.append(e)

# this needs to return True to suppress propagation, as others have said
def __exit__(self, exc_type, exc_val, traceback):
    print 'Attempts', self.attempts
    for e in self.errors:
        print e  # as demo, print them out for good measure!
    return True

现在试一试：

>>> with retry(retries=3) as resource:
...     # if resource is successfully fetched, you can access it as `resource`;
...     # if fetching failed, `resource` will be None
...     print 'I get', resource
I get None
Attempts 3
name 'resource' is not defined
name 'resource' is not defined
name 'resource' is not defined