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 ~]$

Working on a patch for this.

Here's a patch.

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.

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
>>>

This seems like a reasonable request.

There is issue19385 for the inconsistency of dbm.dumb, with a patch.

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

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

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 issue 19385 (since that isn't subject to the beta 1 deadline)

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.

Cool, thanks!

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

nosy: + python-dev
messages: + msg203119

resolution: fixed
stage: patch review -> resolved

messages: + msg200697
nosy: + ncoghlan, rhettinger

messages: + msg200376

stage: needs patch -> patch review