Issue 33106: Deleting a key in a read-only gdbm results in KeyError, not gdbm.error

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

Same problem with 3.6.4, start with

>>> from dbm import gnu as gdbm

then the same incorrect behavior

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?

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

New changeset 4fb0b8bc25c52aae8dcb4353e69c1c88999a9a53 by Xiang Zhang in branch 'master':
bpo-33106: change dbm key deletion error for readonly file from KeyError to dbm.error (#6295)
https://github.com/python/cpython/commit/4fb0b8bc25c52aae8dcb4353e69c1c88999a9a53

messages: + msg314603
versions: + Python 3.8, - Python 3.6