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Python calls an object's __del__ method, even if the
objects __init__ method was unsuccessful (i.e. __init__
terminated via exception.)
Because of this, if there is an error while attempting
to open a Shelf, the destructor is still called, even
though the Shelf is not fully constructed. The code in
shelve.Shelf does not check for this possibility, and
therefor triggers an AttributeError.
Here is an example script which should reproduce the error.
===Begin test.py====l
import shelve
try:
 shelf = shelve.open('non-existant-file', flag='r')
except:
 pass
===End test.py=====
For me, the above script produces the message:
Exception exceptions.AttributeError: "DbfilenameShelf
instance has no attribute 'writeback'" in ignored
I can reproduce this using either Python 2.4.1 or
2.3.5. I took a quick look at the current CVS version
of shelve.py, and it appears to suffer the same problem.

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Is this a bug? As exceptions in __del__ are ignored, this
should do no harm.

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Exceptions thrown in __del__ are not quite complete ignored,
since the fact that they are being "ignored" is reported on
sys.stderr.
The bug in question results in spurious, confusing messages
being issued to sys.stderr. I say it is a bug.

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Fixed in rev. 46946, 46947 (2.4).