Add backend parameter to dbm.open() and shelve.open() for explicit DBM backend selection #137881
Description
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:
- Custom serializers work with some backends but not others (e.g., gdbm type restrictions)
- Cross-platform consistency is needed across different environments
- Predictable behavior is required regardless of which backends are installed
- 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 systemSee: #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