Context Manager getting str instead of AttributeError instance

Peter Otten __peter__ at
Thu Mar 15 20:01:38 CET 2012

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?

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