Unable to read sqlite3 database keys in read-only directory with Python 3.13 with shelve

[jmcarcell]

Bug report

Bug description:

In Python 3.13, sqlite is the new default for creating databases (https://www.python.org/downloads/release/python-3130/). However, it seems that it is not possible to get the keys of the database if the database file is in a read-only folder.

The setup:

mkdir PythonTest
cd PythonTest
python -c "import shelve; shelve.open('db.db', 'c')"
cd ..
chmod 555 PythonTest

Then running

import shelve

with shelve.open('PythonTest/db.db', 'r') as db:
    print(list(db.keys()))

will fail with

Traceback (most recent call last):
  File "/usr/lib/python3.13/dbm/sqlite3.py", line 79, in _execute
    return closing(self._cx.execute(*args, **kwargs))
                   ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^
sqlite3.OperationalError: attempt to write a readonly database

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/read.py", line 5, in <module>
    print(list(db.keys()))
          ~~~~^^^^^^^^^^^
  File "<frozen _collections_abc>", line 854, in __len__
  File "/usr/lib/python3.13/shelve.py", line 99, in __len__
    return len(self.dict)
  File "/usr/lib/python3.13/dbm/sqlite3.py", line 84, in __len__
    with self._execute(GET_SIZE) as cu:
         ~~~~~~~~~~~~~^^^^^^^^^^
  File "/usr/lib/python3.13/dbm/sqlite3.py", line 81, in _execute
    raise error(str(exc))
dbm.sqlite3.error: attempt to write a readonly database

which I believe to be unexpected since the database was opened with the r flag, for read-only (see https://docs.python.org/3/library/dbm.html#dbm.open).

CPython versions tested on:

3.13.2 and 3.13.3

Operating systems tested on:

Linux (Ubuntu 24, Arch Linux)

Linked PRs

• gh-135386: Fix "unable to open database file" errors on readonly DB #135566
• [3.14] gh-135386: Fix "unable to open database file" errors on readonly DB (GH-135566) #138056
• [3.13] gh-135386: Fix "unable to open database file" errors on readonly DB (GH-135566) #138057
• gh-135386: Skip readonly tests for the root user #138058
• [3.13] gh-135386: Skip readonly tests for the root user (GH-138058) #138064

[vstinner]

cc @erlend-aasland

[GeneralK1ng]

Hi,
Thanks for reporting this issue! I have worked on a fix that adds the immutable=1 flag when opening a read-only SQLite DB to avoid WAL/SHM file errors.
I am preparing a PR #135566 and will submit it soon for review.
Looking forward to feedback from the maintainers and community.

Thanks!