| \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. |