# 13.4 `shelve` -- Python object persistence

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`( | filename[,flag='c'[,protocol=`None`[,writeback=`False`]]])

  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 `anydbm.open`.

  By default, version 0 pickles are used to serialize values. The version of the pickle protocol can be specified with the protocol parameter. Changed in version 2.3: The protocol parameter was added.

  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:

`sync`( | )

  Write back all entries in the cache if the shelf was opened with writeback set to True. Also empty the cache and synchronize the persistent dictionary on disk, if feasible. This is called automatically when the shelf is closed with `close()`.