Python Shelve Module
The shelve module implements a persistent, dictionary-like object. It is backed by a database file (using dbm), allowing you to store Python objects that can be pickled.
Importing the Module
import shelveOpening a Shelf
To open a shelf, use shelve.open(). It takes a filename as an argument.
import shelve
# Open a shelf file
with shelve.open('mydata') as shelf:
# ... use the shelf ...
passWriting Data
You can treat the shelf object like a dictionary. Keys must be strings, but values can be any picklable Python object.
import shelve
with shelve.open('mydata') as shelf:
shelf['key1'] = {1, 2, 3}
shelf['key2'] = 'some string'
shelf['key3'] = [10, 20, 30]Reading Data
Reading is also like accessing a dictionary.
import shelve
with shelve.open('mydata') as shelf:
print(shelf['key1'])
# Output: {1, 2, 3}
if 'key2' in shelf:
print(shelf['key2'])Updating Data
By default, shelve does not track modifications to mutable objects stored in the shelf. To ensure changes are saved, you can either re-assign the object or set writeback=True.
Without writeback=True
with shelve.open('mydata') as shelf:
data = shelf['key3']
data.append(40)
shelf['key3'] = data # Explicitly write backWith writeback=True
with shelve.open('mydata', writeback=True) as shelf:
shelf['key3'].append(50) # Automatically savedNote: writeback=True can be slower and consume more memory because it caches all accessed entries.