A “shelf” is a persistent, dictionary-like object. The difference with “dbm” databases is that the values (not the keys!) in a shelf can be essentially arbitrary Python objects — anything that the pickle module can handle. This includes most class instances, recursive data types, and objects containing lots of shared sub-objects. The keys are ordinary strings.
Open a persistent dictionary. The filename specified is the base filename for the underlying database. As a side-effect, an extension may be added to the filename and more than one file may be created. By default, the underlying database file is opened for reading and writing. The optional flag parameter has the same interpretation as the flag parameter of dbm.open().
By default, version 3 pickles are used to serialize values. The version of the pickle protocol can be specified with the protocol parameter.
By default, mutations to persistent-dictionary mutable entries are not automatically written back. If the optional writeback parameter is set to True, all entries accessed are cached in memory, and written back at close time; this can make it handier to mutate mutable entries in the persistent dictionary, but, if many entries are accessed, it can consume vast amounts of memory for the cache, and it can make the close operation very slow since all accessed entries are written back (there is no way to determine which accessed entries are mutable, nor which ones were actually mutated).
Shelve objects support all methods supported by dictionaries. This eases the transition from dictionary based scripts to those requiring persistent storage.
One additional method is supported:
A subclass of collections.MutableMapping which stores pickled values in the dict object.
By default, version 0 pickles are used to serialize values. The version of the pickle protocol can be specified with the protocol parameter. See the pickle documentation for a discussion of the pickle protocols.
If the writeback parameter is True, the object will hold a cache of all entries accessed and write them back to the dict at sync and close times. This allows natural operations on mutable entries, but can consume much more memory and make sync and close take a long time.
To summarize the interface (key is a string, data is an arbitrary object):
import shelve d = shelve.open(filename) # open -- file may get suffix added by low-level # library d[key] = data # store data at key (overwrites old data if # using an existing key) data = d[key] # retrieve a COPY of data at key (raise KeyError if no # such key) del d[key] # delete data stored at key (raises KeyError # if no such key) flag = key in d # true if the key exists klist = d.keys() # a list of all existing keys (slow!) # as d was opened WITHOUT writeback=True, beware: d['xx'] = range(4) # this works as expected, but... d['xx'].append(5) # *this doesn't!* -- d['xx'] is STILL range(4)!!! # having opened d without writeback=True, you need to code carefully: temp = d['xx'] # extracts the copy temp.append(5) # mutates the copy d['xx'] = temp # stores the copy right back, to persist it # or, d=shelve.open(filename,writeback=True) would let you just code # d['xx'].append(5) and have it work as expected, BUT it would also # consume more memory and make the d.close() operation slower. d.close() # close it