Combining pickle and anydbm to form shelve
Now that we've talked about both pickle and anydbm, we can now talk about marrying the two with shelve. The shelve module brings pickle functionality to dbm-style databases, allowing us to store massive data entries in dictionary form without the expected performance hit. It's the middle option between a full-on database implementation and simple flat file storage.
We create a new shelf like so:
import shelve
s = shelve.open('shelving', flag='c') # the open() method is a wrapper around anydbm.open()
s['name'] = "Joe"
s.close()
A shelf is created by using the shelve module's open() method, a wrapper around anydbm.open(). Once we have that shelf, we can treat it much like we would a dictionary. We save a value by specifying a dictionary key/value pair. Finally, we close the filestream.
Reading data from the shelf is equally simple:
import shelve
s = shelve.open('shelving', flag='r')
print s['name']
s.close()
By default, a shelf does not save modifications to volatile object structure. We see this in the following example:
import shelve
s = shelve.open('shelving', flag='c') # the open() method is a wrapper around anydbm.open()
s['name'] = {}
s.close()
s = shelve.open('shelving')
s['name']['fname'] = 'Joe'
s['name']['lname'] = 'Mosby'
s.close()
s = shelve.open('shelving')
print s['name'] # will print {}
s.close()
However, if we enable writeback when we open the shelf, we can make direct changes to an object. (warning: performance will take a hit as this will open a potentially massive object in memory)
import shelve
s = shelve.open('shelving', flag='c') # the open() method is a wrapper around anydbm.open()
s['name'] = {}
s.close()
s = shelve.open('shelving', writeback=True)
s['name']['fname'] = 'Joe'
s['name']['lname'] = 'Mosby'
s.close()
s = shelve.open('shelving')
print s['name'] # will print {'fname': 'Joe', 'lname': 'Mosby'}
s.close()
And that's where we stand with shelve. Not too much to it - just a little persistence library built for a very niche database format.