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\section{Built-in module \sectcode{shelve}}
\stmodindex{shelve}
\stmodindex{pickle}
\bimodindex{dbm}
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
\code{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 (\code{key} is a string, \code{data} is an
arbitrary object):
\begin{verbatim}
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
\end{verbatim}
Dependent on the implementation, closing a persistent dictionary may
or may not be necessary to flush changes to disk.
Note: \code{shelve} does not support {\em concurrent} access to
shelved objects. Two programs should not try to simultaneously access
the same shelf.
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