Add backend parameter to dbm.open() and shelve.open() for explicit DBM backend selection

Feature or enhancement

Proposal:

Summary

Add an optional backend parameter to dbm.open() and shelve.open() functions to allow explicit selection of DBM backend implementations, improving compatibility and predictability across different systems.

Motivation

Currently, Python's DBM module automatically selects an available backend implementation in priority order (dbm.sqlite3, dbm.gnu, dbm.ndbm, dbm.dumb). This can cause compatibility issues when:

1. Custom serializers work with some backends but not others (e.g., gdbm type restrictions)
2. Cross-platform consistency is needed across different environments
3. Predictable behavior is required regardless of which backends are installed
4. Testing needs to validate behavior with specific backends

Real-world example

# This works with dbm.sqlite3 but fails with dbm.gnu
def custom_serializer(obj, protocol):
    if protocol == 5 and isinstance(obj, bytearray):
        return obj  # Causes "gdbm mappings have bytes or string indices only"
    return pickle.dumps(obj, protocol)

# Currently no way to ensure consistent backend
with shelve.open('data.db', serializer=custom_serializer) as shelf:
    shelf['key'] = bytearray(b'data')  # May fail depending on system

See: https://github.com/python/cpython/issues/137829

Proposed Solution

Add an optional backend parameter to both functions:

# dbm.open()
dbm.open(file, flag='r', mode=0o666, backend=None)

# shelve.open() 
shelve.open(filename, flag='c', protocol=None, writeback=False, 
            backend=None, *, serializer=None, deserializer=None)

Usage Examples:

import dbm
import shelve

# Explicit backend selection
with dbm.open('data.db', 'c', backend='dbm.dumb') as db:
    db[b'key'] = b'value'

# Cross-platform consistency 
with shelve.open('data.shelf', backend='dbm.sqlite3') as shelf:
    shelf['key'] = complex_object

# Testing with specific backends
for backend in ['dbm.gnu', 'dbm.ndbm', 'dbm.dumb']:
    with shelve.open(f'test_{backend}.db', backend=backend) as shelf:
        test_serialization_compatibility(shelf)

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-137882
• gh-138007

Start by examining the dbm.open() and shelve.open() entry points and the existing backend-selection behavior described in the issue. Review linked PRs gh-137882 and gh-138007 before starting, since they indicate work has already moved elsewhere. Done would include an agreed backend-selection API and coverage for explicit backend choices across the supported implementations.