Shelve references globals in its __del__ method

[PeterDavies]

BPO	11640
Nosy	@durban, @akheron, @berkerpeksag
Files	shelve_del_fix.diff: Fix which stores the globals in the shelf

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2016-09-12.02:43:11.820>
created_at = <Date 2011-03-22.22:15:42.956>
labels = ['type-bug', 'library']
title = 'Shelve references globals in its __del__ method'
updated_at = <Date 2016-09-12.02:43:11.818>
user = 'https://bugs.python.org/PeterDavies'

bugs.python.org fields:

activity = <Date 2016-09-12.02:43:11.818>
actor = 'berker.peksag'
assignee = 'none'
closed = True
closed_date = <Date 2016-09-12.02:43:11.820>
closer = 'berker.peksag'
components = ['Library (Lib)']
creation = <Date 2011-03-22.22:15:42.956>
creator = 'Peter.Davies'
dependencies = []
files = ['21345']
hgrepos = []
issue_num = 11640
keywords = ['needs review']
message_count = 4.0
messages = ['131793', '141436', '141524', '275925']
nosy_count = 5.0
nosy_names = ['stutzbach', 'daniel.urban', 'Peter.Davies', 'petri.lehtinen', 'berker.peksag']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue11640'
versions = ['Python 2.7', 'Python 3.2', 'Python 3.3', 'Python 3.4']

[PeterDavies]

Shelf.__setitem__ (which is called from __del__ when writeback is enabled) references globals. This was causing exceptions on interpreter shutdown (due to another exception) for me.

I have attached a patch which stores the relevant globals in the Shelf object.

[akheron]

The patch looks good to me. Is there any way this could be tested in the test suite? How to simulate interpreter shutdown?

[stutzbach]

Could you add a comment above the lines defining self._BytesIO, describing why they're being set? Someone else might see them as unnecessary and rip them out if there's no explanation.

Can a test launch Python in a subprocess, set up the appropriate data structure, let the subprocess Python shutdown, then check the subprocess's stderr for the error?

It's a little convoluted, but perhaps it could be built into a general utility function that could be used to test __del__ methods in other modules, too.

Not sure that it's worth the effort though.

[berkerpeksag]

Since PEP-442 has been implemented in 3.4 and e826940911c8 made Shelve.close() more robust, I think this is no longer an issue.