With statement destructor to catch init exceptions

Question

I use a with statement with the following class.

def __init__(self):
    ...

def __enter__(self):
    return self

def __exit__(self, type, value, traceback):
    print "EXIT Shutting the SDK down"
    ret = self.sdkobject.ShutDown()
    self.error_check(ret)

This catches any error that occur when I am using the object of the class and safely shuts down the SDK that I am using. However, it catch problems when the class is still initializing. I have recently found the "del" function which neatly solves this problem. However, it can't be used in conjunction with the exit function (as the with statement evokes the exit and the del gets an exception). How can I set up a destructor using a with statemtent, which will catch failures even during initialization?

You mean that exceptions in `__init__` need to trigger a shutdown too? — Martijn Pieters, Jan 21 '14 at 17:08

score 2 · Answer 1 · answered Jan 21 '14 at 17:11

Exceptions in the __init__ need to be dealt with directly in that method:

class YourContextManager(object):
    sdkobject = None

    def __init__(self):
        try:
             self._create_sdk_object()
        except Exception:
             if self.sdkobject is not None:
                 self.sdkobject.ShutDown()
             raise

    def _create_sdk_object(self):
        self.sdkobject = SomeSDKObject()
        self.sdkobject.do_something_that_could_raise_an_exception()

    def __enter__(self):
        return self

    def __exit__(self, type, value, traceback):
        print "EXIT Shutting the SDK down"
        ret = self.sdkobject.ShutDown()
        self.error_check(ret)

Note that the exception is re-raised; you want to give the consumer of the context manager an opportunity to handle the failure to create a context manager.

score 1 · Accepted Answer · answered Jan 21 '14 at 17:09

1

Create a separate shutdown function that gets called in the try/except block of the __init__ and wherever else you need a proper shutdown.

answered Jan 21 '14 at 17:09

jramirez

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Are try except statements OK in the __init__ class? – Marmstrong Jan 21 '14 at 17:10
@Marmstrong: Yes. Why wouldn't they be? – user2357112 Jan 21 '14 at 17:11
Yeah sprinkle them away wherever needed. – jramirez Jan 21 '14 at 17:13

score 0 · Answer 3 · answered Jan 21 '14 at 17:09

0

Catch the exception in __init__ and handle it. __del__ is unnecessary.

answered Jan 21 '14 at 17:09

user2357112

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With statement destructor to catch init exceptions

3 Answers3