Deleting a key in a read-only gdbm results in KeyError, not gdbm.error

[sam-s]

BPO	33106
Nosy	@zhangyangyu, @sam-s
PRs	bpo-33106: change dbm key deletion error for readonly file from KeyError to dbm.error #6295

^{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 2018-12-12.13:05:50.142>
created_at = <Date 2018-03-19.21:44:11.062>
labels = ['3.8', 'type-feature', 'library']
title = 'Deleting a key in a read-only gdbm results in KeyError, not gdbm.error'
updated_at = <Date 2018-12-12.13:05:50.141>
user = 'https://github.com/sam-s'

bugs.python.org fields:

activity = <Date 2018-12-12.13:05:50.141>
actor = 'xiang.zhang'
assignee = 'none'
closed = True
closed_date = <Date 2018-12-12.13:05:50.142>
closer = 'xiang.zhang'
components = ['Library (Lib)']
creation = <Date 2018-03-19.21:44:11.062>
creator = 'sam-s'
dependencies = []
files = []
hgrepos = []
issue_num = 33106
keywords = ['patch']
message_count = 5.0
messages = ['314119', '314120', '314603', '319062', '331694']
nosy_count = 2.0
nosy_names = ['xiang.zhang', 'sam-s']
pr_nums = ['6295']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue33106'
versions = ['Python 3.8']

[sam-s]

deleting a key from a read-only gdbm should be gdbm.error, not KeyError:

>>> import gdbm
>>> db = gdbm.open("foo","n")   # create new
>>> db["a"] = "b"
>>> db.close()
>>> db = gdbm.open("foo","r")   # read only
>>> db["x"] = "1"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
gdbm.error: Reader can't store    # correct
>>> db["a"]
'b'
>>> del db["a"]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'a'    # WRONG!  should be the same as above

[sam-s]

Same problem with 3.6.4, start with

>> from dbm import gnu as gdbm

then the same incorrect behavior

[zhangyangyu]

I like this idea. But this is a behavior change so I think it could only be applied to master branch. But there is a problem, except dbm.gnu and dbm.ndbm, we also get dbm.dumb. It already raises a ValueError when deleting a key in readonly mode. Does it need to be changed to raise dbm.dumb.error to achieve consistency?

[sam-s]

I think consistency would be nice -- so, yes, dbm.dumb.error instead of ValueError (or at least a subtype of dbm.dumb.error).
Thanks!

[zhangyangyu]

New changeset 4fb0b8b by Xiang Zhang in branch 'master':
bpo-33106: change dbm key deletion error for readonly file from KeyError to dbm.error (bpo-6295)
4fb0b8b