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From: Peter Otten <__peter__@web.de>
Subject: RE: Context Manager getting str instead of AttributeError instance
Date: Thu, 15 Mar 2012 21:38:22 +0100
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Xref: csiph.com comp.lang.python:21712

Prasad, Ramit wrote:

>> Prasad, Ramit wrote:
>> 
>> > So I have a context manager used to catch errors
>> >
>> > def __exit__( self, exceptionClass, exception, tracebackObject ):
>> >     if isinstance( exception, self.exceptionClasses ):
>> >          #do something here
>> >
>> > Normally exception would be the exception instance, but for
>> > AttributeError it seems to be a string instead.
>> 
>> I don't think so:
>> 
>> >>> class A(object):
>> ...     def __enter__(self): return self
>> ...     def __exit__(self, *args): print args
>> ...
>> >>> with A() as a:
>> ...     a.x
>> ...
>> (<type 'exceptions.AttributeError'>, AttributeError("'A' object has no
>> attribute 'x'",), <traceback object at 0x7f57b70f22d8>)
>> Traceback (most recent call last):
>>   File "<stdin>", line 2, in <module>
>> AttributeError: 'A' object has no attribute 'x'
>> 
>> > 1) Why is AttributeError different than the other built-ins
>> > in this respect?
>> > 2) Are there other standard errors like this (I know
>> > that SystemExit is different as well)?
>> > 3) Taking into account that I want to include subclasses of
>> > classes listed in self.exceptionClasses, Is there a better check I can
>> > use?
> 
> Not sure why mine behaves differently:
> Python 2.6.6 (r266:84292, Dec 17 2010, 12:36:53) [MSC v.1500 32 bit
> (Intel)] on win32
>>>> 
>>>> class A(object):
> ...     def __enter__(self): return self
> ...     def __exit__(self, *args): print args
> ...
>>>> with A() as a:
> ...     a.x
> ...
> (<type 'exceptions.AttributeError'>, "'A' object has no attribute 'x'",
> <traceback object at 0x1817F648>) AttributeError: 'A' object has no
> attribute 'x'
> 
> As you can see, I am getting a string while you are not.

Ah, it's a bug:

http://bugs.python.org/issue7853

Unfortunately it is to severe to fix in a bugfix release. You could work 
around it with an issubclass() test on the first argument.