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60 lines
2.1 KiB
TeX
60 lines
2.1 KiB
TeX
\section{Standard Module \sectcode{shelve}}
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\stmodindex{shelve}
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\stmodindex{pickle}
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\bimodindex{dbm}
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\bimodindex{gdbm}
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A ``shelf'' is a persistent, dictionary-like object. The difference
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with ``dbm'' databases is that the values (not the keys!) in a shelf
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can be essentially arbitrary Python objects --- anything that the
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\code{pickle} module can handle. This includes most class instances,
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recursive data types, and objects containing lots of shared
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sub-objects. The keys are ordinary strings.
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To summarize the interface (\code{key} is a string, \code{data} is an
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arbitrary object):
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\begin{verbatim}
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import shelve
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d = shelve.open(filename) # open, with (g)dbm filename -- no suffix
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d[key] = data # store data at key (overwrites old data if
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# using an existing key)
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data = d[key] # retrieve data at key (raise KeyError if no
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# such key)
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del d[key] # delete data stored at key (raises KeyError
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# if no such key)
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flag = d.has_key(key) # true if the key exists
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list = d.keys() # a list of all existing keys (slow!)
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d.close() # close it
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\end{verbatim}
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Restrictions:
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\begin{itemize}
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\item
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The choice of which database package will be used (e.g. dbm or gdbm)
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depends on which interface is available. Therefore it isn't safe to
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open the database directly using dbm. The database is also
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(unfortunately) subject to the limitations of dbm, if it is used ---
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this means that (the pickled representation of) the objects stored in
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the database should be fairly small, and in rare cases key collisions
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may cause the database to refuse updates.
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\item
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Dependent on the implementation, closing a persistent dictionary may
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or may not be necessary to flush changes to disk.
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\item
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The \code{shelve} module does not support {\em concurrent} read/write
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access to shelved objects. (Multiple simultaneous read accesses are
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safe.) When a program has a shelf open for writing, no other program
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should have it open for reading or writing. \UNIX{} file locking can
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be used to solve this, but this differs across \UNIX{} versions and
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requires knowledge about the database implementation used.
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\end{itemize}
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