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Title:, flag="n") raises dbm.error if file exists and is rejected by whichdb
Type: behavior Stage: resolved
Components: Library (Lib) Versions: Python 2.7
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: brian.curtin Nosy List: brian.curtin, denversc, eric.araujo, python-dev, r.david.murray
Priority: normal Keywords: patch

Created on 2011-03-14 06:42 by denversc, last changed 2022-04-11 14:57 by admin. This issue is now closed.

File name Uploaded Description Edit
dbm_open_n_flag_error_invalid_file_fix_v1.patch denversc, 2011-03-14 06:42 Proposed Fix (version 1) review
issue11491.diff brian.curtin, 2011-03-14 17:40 review
Messages (8)
msg130791 - (view) Author: Denver Coneybeare (denversc) * Date: 2011-03-14 06:42 with flag="n" raises dbm.error if the given file exists but whichdb doesn't recognize it.  In the documentation for the "n" flag is documented to "Always create a new, empty database, open for reading and writing".  To me, this implies that if the file exists it will unconditionally be overwritten with a newly-created database, irrespective of its contents.

The code below illustrates a scenario (and indeed the scenario that I ran into) where, flag="n") will throw dbm.error when it should just blow away the existing file and create a new, empty database:

import dbm
open("test.db", "w").close() # create empty file"test.db", flag="n")

The cause of the exception is that within there is a call to whichdb to determine the file type.  The fix would be to skip this whichdb check if the "n" flag is specified.

I don't think that this change will cause backward compatibility issues, since I find it hard to believe that existing applications would rely on this exception being raised in this scenario.  However, to *guarantee* no compatibility break an alternate fix could leave the current behavior of the "n" flag and introduce a new flag, perhaps "o" for "overwrite", with this "unconditional overwrite" behavior.

A proposed patch is attached: dbm_open_n_flag_error_invalid_file_fix_v1.patch
msg130865 - (view) Author: Brian Curtin (brian.curtin) * (Python committer) Date: 2011-03-14 17:40
Attached is a slightly updated version of the patch. If the assertEqual for any reason were to fail, the file wouldn't be closed, leading to a ResourceWarning. That'll work on 3.2+, but if this is backported consideration will have to be made there (try/finally, probably).

Otherwise, in concept I think this patch is alright.
msg130869 - (view) Author: Denver Coneybeare (denversc) * Date: 2011-03-14 18:21
Looks good to me.  I thought the same thing about the file not being closed on error, but all of the other tests in the file also suffer from that problem, so I just followed the convention set out by the other tests.  Maybe if you eventually commit this change, you could improve this behavior in the other tests as well.
msg130871 - (view) Author: Éric Araujo (eric.araujo) * (Python committer) Date: 2011-03-14 18:28
Fortunately, addCleanup exists in 2.7 and 3.1.
msg130879 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2011-03-14 19:36
New changeset f8d603a2a0af by briancurtin in branch '3.1':
Fix #11491. When was called with a file which already exists and
msg130893 - (view) Author: Roundup Robot (python-dev) (Python triager) Date: 2011-03-14 20:36
New changeset ec8d64396be9 by briancurtin in branch '3.2':
Fix #11491. When was called with a file which already exists and

New changeset 0515206e36ed by briancurtin in branch 'default':
Fix #11491. When was called with a file which already exists and
msg130914 - (view) Author: Brian Curtin (brian.curtin) * (Python committer) Date: 2011-03-14 21:51
The 3.x side of things is taken care of. Still need to see if there is any issue on 2.7 where things are organized differently and dbm is in C.
msg228412 - (view) Author: R. David Murray (r.david.murray) * (Python committer) Date: 2014-10-03 23:58
Given that this has some backward compatibility implications (files getting overwritten that previously wouldn't have been), I think we should just leave 2.7 alone, so I'm closing this.
Date User Action Args
2022-04-11 14:57:14adminsetgithub: 55700
2014-10-03 23:58:29r.david.murraysetstatus: open -> closed

versions: - Python 3.1, Python 3.2, Python 3.3
nosy: + r.david.murray

messages: + msg228412
resolution: fixed
stage: commit review -> resolved
2011-03-14 21:51:53brian.curtinsetassignee: brian.curtin
messages: + msg130914
nosy: eric.araujo, brian.curtin, denversc, python-dev
stage: commit review
2011-03-14 20:36:53python-devsetnosy: eric.araujo, brian.curtin, denversc, python-dev
messages: + msg130893
2011-03-14 19:36:09python-devsetnosy: + python-dev
messages: + msg130879
2011-03-14 18:28:52eric.araujosetnosy: + eric.araujo
messages: + msg130871
2011-03-14 18:21:01denverscsetnosy: brian.curtin, denversc
messages: + msg130869
2011-03-14 17:40:27brian.curtinsetfiles: + issue11491.diff
nosy: brian.curtin, denversc
messages: + msg130865
2011-03-14 15:27:53denverscsetnosy: + brian.curtin

versions: + Python 3.1, Python 2.7, Python 3.2
2011-03-14 06:42:42denversccreate