dbm.open should be a context manager

[kousu]

BPO	19282
Nosy	@rhettinger, @terryjreedy, @ncoghlan, @kousu, @PCManticore
Files	dbm.patch dbm1.patch

^{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 = 'https://github.com/ncoghlan'
closed_at = <Date 2013-11-17.06:00:20.899>
created_at = <Date 2013-10-18.07:45:10.904>
labels = ['type-feature', 'library']
title = 'dbm.open should be a context manager'
updated_at = <Date 2014-03-08.17:54:18.487>
user = 'https://github.com/kousu'

bugs.python.org fields:

activity = <Date 2014-03-08.17:54:18.487>
actor = 'python-dev'
assignee = 'ncoghlan'
closed = True
closed_date = <Date 2013-11-17.06:00:20.899>
closer = 'python-dev'
components = ['Library (Lib)']
creation = <Date 2013-10-18.07:45:10.904>
creator = 'kousu'
dependencies = []
files = ['32203', '32215']
hgrepos = []
issue_num = 19282
keywords = ['patch']
message_count = 13.0
messages = ['200191', '200232', '200241', '200336', '200376', '200697', '201545', '201818', '203119', '203120', '203121', '203123', '212941']
nosy_count = 7.0
nosy_names = ['rhettinger', 'terry.reedy', 'ncoghlan', 'kousu', 'Arfrever', 'Claudiu.Popa', 'python-dev']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue19282'
versions = ['Python 3.4']

[kousu]

This code doesn't work. I think it should.

import dbm
with dbm.open("what is box.db", "c") as db:
   db["Bpoind"] = Boing

Indeed, there is nothing supporting PEP-343 for dbm on my system:
[kousu@galleon ~]$ grep -r __exit__ /usr/lib/python3.3/dbm
[kousu@galleon ~]$

[PCManticore]

Working on a patch for this.

[PCManticore]

Here's a patch.

[terryjreedy]

I agree with the revised title. Test_context_manager() should also test that db is actually closed after the with statement. I presume you can use self.assertRaises and try to do something with db that will fail if it is properly closed.

        with self.assertRaises(<NotOpenError>):
            db['a'] = 'a'  # or whatever

I haven't looked at the C code.

[PCManticore]

Attached patch checks that the db is actually closed after the with. Also, I noticed that dbm.dumb doesn't behave like the rest of the variants, as seen in the following:

>>> import dbm.dumb as d
>>> db = d.open('test.dat', 'c')
>>> db.close()
>>> db.keys()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/tank/libs/cpython/Lib/dbm/dumb.py", line 212, in keys
    return list(self._index.keys())
AttributeError: 'NoneType' object has no attribute 'keys'
>>>

vs

>>> import dbm.gnu as g
>>> db = g.open('test.dat', 'c')
>>> db.close()
>>> db.keys()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
_gdbm.error: GDBM object has already been closed
>>>

[rhettinger]

This seems like a reasonable request.

[PCManticore]

There is bpo-19385 for the inconsistency of dbm.dumb, with a patch.

[ncoghlan]

Patch looks reasonable to me - if nobody beats me to it, I'll commit this before beta 1 :)

[python-dev]

New changeset c2f1bb56760d by Nick Coghlan in branch 'default':
Close bpo-19282: Native context management in dbm
http://hg.python.org/cpython/rev/c2f1bb56760d

[ncoghlan]

Thanks Claudiu!

Additional tweaks in the committed version:

• added a paragraph to the docs about the with statement support and moved the versionchanged note there
• changed the example in the docs to use a with statement
• added a basic test that the dumbdbm impl was actually closed, with a comment referencing bpo-19385 (since that isn't subject to the beta 1 deadline)

[ncoghlan]

I also changed the signature of the enter/exit methods to just use PyObject * (rather than dbmobject *), since that allowed a bunch of casts to be removed and eliminated a couple of compiler warnings about type mismatches.

[PCManticore]

Cool, thanks!

[python-dev]

New changeset 200207e50cbf by R David Murray in branch 'default':
whatsnew: dbm.open is context manager. (bpo-19282)
http://hg.python.org/cpython/rev/200207e50cbf