76 lines
2.8 KiB
TeX
76 lines
2.8 KiB
TeX
\section{\module{shelve} ---
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Python object persistency}
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\declaremodule{standard}{shelve}
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\modulesynopsis{Python object persistency.}
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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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\refmodule{pickle} module can handle. This includes most class
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instances, recursive data types, and objects containing lots of shared
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sub-objects. The keys are ordinary strings.
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\refstmodindex{pickle}
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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
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(e.g. \refmodule{dbm} or \refmodule{gdbm}) depends on which interface
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is available. Therefore it is not safe to open the database directly
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using \refmodule{dbm}. The database is also (unfortunately) subject
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to the limitations of \refmodule{dbm}, if it is used --- this means
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that (the pickled representation of) the objects stored in the
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database should be fairly small, and in rare cases key collisions may
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cause the database to refuse updates.
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\refbimodindex{dbm}
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\refbimodindex{gdbm}
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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 \module{shelve} module does not support \emph{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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\begin{seealso}
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\seemodule{anydbm}{Generic interface to \code{dbm}-style databases.}
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\seemodule{dbhash}{BSD \code{db} database interface.}
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\seemodule{dbm}{Standard \UNIX{} database interface.}
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\seemodule{dumbdbm}{Portable implementation of the \code{dbm} interface.}
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\seemodule{gdbm}{GNU database interface, based on the \code{dbm} interface.}
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\seemodule{pickle}{Object serialization used by \module{shelve}.}
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\seemodule{cPickle}{High-performance version of \module{pickle}.}
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\end{seealso}
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