Beginning Python - From Novice to Professional

232 CHAPTER 10 ■ BATTERIES INCLUDED
>>> import shelve
>>> s = shelve.open('test.dat')
>>> s['x'] = ['a', 'b', 'c']
>>> s['x'].append('d')
>>> s['x']
['a', 'b', 'c']
Where did the 'd' go?
The explanation is simple: When you look up an element in a shelf object, the object is
reconstructed from its stored version; and when you assign an element to a key, it is stored.
What happened in the preceding example was the following:
1. The list ['a', 'b', 'c'] was stored in s under the key 'x'.
2. The stored representation was retrieved, a new list was constructed from it, and 'd' was
appended to the copy. This modified version was not stored!
3. Finally, the original is retrieved again—without the 'd'.
To correctly modify an object that is stored using the shelve module, you must bind a
temporary variable to the retrieved copy, and then store the copy again after it has been modified:
>>> temp = s['x']
>>> temp.append('d')
>>> s['x'] = temp
>>> s['x']
['a', 'b', 'c', 'd']
Thanks to Luther Blissett for pointing this out.
From Python 2.4 onward, there is another way around this problem: Setting the writeback
parameter of the open function to true. If you do, all of the data structures that you read from or
assign to the shelf will be kept around in memory (cached) and only written back to disk when
you close the shelf. If you’re not working with huge data, and you don’t want to worry about
these things, setting writeback to true (and making sure you close your shelf at the end) may be
a good idea.
Example
Listing 10-8 shows a simple database application that uses the shelve module.
Listing 10-8. A Simple Database Application
# database.py
import sys, shelve
def store_person(db):
"""
Query user for data and store it in the shelf object
"""