[issue13896] Make shelf instances work with 'with' as context managers

[Andrew Svetlov]

Andrew Svetlov added the comment:

Filip, please describe why you prevent closing if writeback attribute is not present.
I see comment for __del__ related to #1339007, but at __exit__ call __enter__ has called already and __init__ has finished definitely with success.
Also I would to see reference to http://bugs.python.org/issue1339007 in __del__ comment to cleaner description of issue.

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<http://bugs.python.org/issue13896>
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