The shelve module can be used as a simple persistent storage option for Python objects when a relational database is not required. The shelf is accessed by keys, just as with a dictionary. The values are pickled and written to a database created and managed by dbm.

Creating a new Shelf



In [1]:

    
import shelve
with shelve.open('test_shelf.db') as s:
     s['key1'] = {
        'int': 10,
        'float': 9.5,
        'string': 'Sample data',
    }



In [2]:

    
import shelve

with shelve.open('test_shelf.db') as s:
    existing = s['key1']

print(existing)









    



{'int': 10, 'float': 9.5, 'string': 'Sample data'}



In [3]:

    
import dbm
import shelve

with shelve.open('test_shelf.db', flag='r') as s:
    print('Existing:', s['key1'])
    try:
        s['key1'] = 'new value'
    except dbm.error as err:
        print('ERROR: {}'.format(err))









    



Existing: {'int': 10, 'float': 9.5, 'string': 'Sample data'}
ERROR: cannot add item to database

Write Back



In [4]:

    
import shelve

with shelve.open('test_shelf.db') as s:
    print(s['key1'])
    s['key1']['new_value'] = 'this was not here before'

with shelve.open('test_shelf.db', writeback=True) as s:
    print(s['key1'])









    



{'int': 10, 'float': 9.5, 'string': 'Sample data'}
{'int': 10, 'float': 9.5, 'string': 'Sample data'}



In [5]:

    
import shelve
import pprint

with shelve.open('test_shelf.db', writeback=True) as s:
    print('Initial data:')
    pprint.pprint(s['key1'])

    s['key1']['new_value'] = 'this was not here before'
    print('\nModified:')
    pprint.pprint(s['key1'])

with shelve.open('test_shelf.db', writeback=True) as s:
    print('\nPreserved:')
    pprint.pprint(s['key1'])









    



Initial data:
{'float': 9.5, 'int': 10, 'string': 'Sample data'}

Modified:
{'float': 9.5,
 'int': 10,
 'new_value': 'this was not here before',
 'string': 'Sample data'}

Preserved:
{'float': 9.5,
 'int': 10,
 'new_value': 'this was not here before',
 'string': 'Sample data'}