Python特性（十四）：context manager的__exit__方法

在context manager对象的__exit__方法中，共有三个参数，保存with语句在运行过程中的异常信息。其中：

第1个参数保存异常类型；
第2个参数保存异常对象的值；
第3个参数保存异常的traceback信息。

请看下面的代码。

class MyContextManager:

    def __enter__(self):
        print "Entering my conext manager"
        

    def __exit__(self, exc_t, exc_v, traceback):
        print "Exiting my conext manager"
        print "Exception type:"
        print exc_t
        print "Exception value:"
        print exc_v
        print "Exception traceback:"
        print traceback
  

with MyContextManager():
    print "In main block"
    raise Exception("Exception raised in main block")

其输出如下：

Entering my conext manager
In main block
Exiting my conext manager
Exception type:
<type 'exceptions.Exception'>
Exception value:
Exception raised in main block
Exception traceback:
<traceback object at 0x0000000002AB0848>
Traceback (most recent call last):
  File "test.txt", line 30, in <module>
    raise Exception("Exception raised in main block")
Exception: Exception raised in main block

需要指出的是，只有在with语句块中抛出的异常才会交给__exit__方法处理。如果在contextmanager的初始化方法和__enter__方法中抛出的异常，并不会交给__exit__方法处理，而是直接向外抛出。看下面的例子。

class MyContextManager:

    def __init__(self):
        print "Initializing my context manager"

    def __enter__(self):
        print "Entering my conext manager"
        raise Exception("Exception raised in __enter__")

    def __exit__(self, exc_t, exc_v, traceback):
        print "Exiting my conext manager"
        print "Exception type:"
        print exc_t
        print "Exception value:"
        print exc_v
        print "Exception traceback:"
        print traceback
  

with MyContextManager():
    print "In main block"

其输出如下：

Initializing my context manager
Entering my conext manager
Traceback (most recent call last):
  File "test.txt", line 31, in <module>
    with MyContextManager():
  File "test.txt", line 19, in __enter__
    raise Exception("Exception raised in __enter__")
Exception: Exception raised in __enter__

注意：上面输出中虽然也打印了异常的traceback信息，但却不是__exit__方法打印的。

既然上面示例的异常没有被__exit__方法处理，那么该异常就可以被主方法捕获。看下面的例子。

class MyContextManager:

    def __init__(self):
        print "Initializing my context manager"

    def __enter__(self):
        print "Entering my conext manager"
        raise Exception("Exception raised in __enter__")

    def __exit__(self, exc_t, exc_v, traceback):
        print "Exiting my conext manager"
        print "Exception type:"
        print exc_t
        print "Exception value:"
        print exc_v
        print "Exception traceback:"
        print traceback
  
try:
    with MyContextManager():
        print "In main block"
except Exception as e:
    print "An exception caught:"
    print e

输出如下：

Initializing my context manager
Entering my conext manager
An exception caught:
Exception raised in __enter__

Python特性（十四）：context manager的exit方法

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