# 3.7 Standard Module `shelve`

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.

To summarize the interface (`key` is a string, `data` is an arbitrary object):

    import shelve

    d = shelve.open(filename) # open, with (g)dbm filename -- no suffix

    d[key] = data   # store data at key (overwrites old data if
                    # using an existing key)
    data = d[key]   # retrieve data at key (raise KeyError if no
                    # such key)
    del d[key]      # delete data stored at key (raises KeyError
                    # if no such key)
    flag = d.has_key(key)   # true if the key exists
    list = d.keys() # a list of all existing keys (slow!)

    d.close()       # close it

Restrictions:

- The choice of which database package will be used (e.g. `dbm` or `gdbm`) depends on which interface is available. Therefore it isn't safe to open the database directly using `dbm`. The database is also (unfortunately) subject to the limitations of `dbm`, if it is used -- this means that (the pickled representation of) the objects stored in the database should be fairly small, and in rare cases key collisions may cause the database to refuse updates.
- Dependent on the implementation, closing a persistent dictionary may or may not be necessary to flush changes to disk.
- The `shelve` module does not support concurrent read/write access to shelved objects. (Multiple simultaneous read accesses are safe.) When a program has a shelf open for writing, no other program should have it open for reading or writing. Unix file locking can be used to solve this, but this differs across Unix versions and requires knowledge about the database implementation used.