sue['pay'] *= 1.10
suefile = open('sue.pkl', 'wb')
pickle.dump(sue, suefile)
suefile.close()
Here are our file-per-record scripts in action; the results are about the same as in the
prior section, but database keys become real filenames now. In a sense, the filesystem
becomes our top-level dictionary—filenames provide direct access to each record.
...\PP4E\Preview> python make_db_pickle_recs.py
...\PP4E\Preview> python dump_db_pickle_recs.py
bob.pkl =>
{'pay': 30000, 'job': 'dev', 'age': 42, 'name': 'Bob Smith'}
sue.pkl =>
{'pay': 40000, 'job': 'hdw', 'age': 45, 'name': 'Sue Jones'}
tom.pkl =>
{'pay': 0, 'job': None, 'age': 50, 'name': 'Tom'}
Sue Jones...\PP4E\Preview> python update_db_pickle_recs.py
...\PP4E\Preview> python dump_db_pickle_recs.py
bob.pkl =>
{'pay': 30000, 'job': 'dev', 'age': 42, 'name': 'Bob Smith'}
sue.pkl =>
{'pay': 44000.0, 'job': 'hdw', 'age': 45, 'name': 'Sue Jones'}
tom.pkl =>
{'pay': 0, 'job': None, 'age': 50, 'name': 'Tom'}
Sue JonesUsing Shelves
Pickling objects to files, as shown in the preceding section, is an optimal scheme in
many applications. In fact, some applications use pickling of Python objects across
network sockets as a simpler alternative to network protocols such as the SOAP and
XML-RPC web services architectures (also supported by Python, but much heavier than
pickle).
Moreover, assuming your filesystem can handle as many files as you’ll need, pickling
one record per file also obviates the need to load and store the entire database for each
update. If we really want keyed access to records, though, the Python standard library
offers an even higher-level tool: shelves.
Shelves automatically pickle objects to and from a keyed-access filesystem. They behave
much like dictionaries that must be opened, and they persist after each program exits.
Because they give us key-based access to stored records, there is no need to manually
manage one flat file per record—the shelve system automatically splits up stored re-
cords and fetches and updates only those records that are accessed and changed. In
this way, shelves provide utility similar to per-record pickle files, but they are usually
easier to code.
Step 2: Storing Records Persistently | 23