mirror of https://github.com/python/cpython
110 lines
4.6 KiB
TeX
110 lines
4.6 KiB
TeX
\section{\module{shelve} ---
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Python object persistence}
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\declaremodule{standard}{shelve}
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\modulesynopsis{Python object persistence.}
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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 -- file may get suffix added by low-level
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# library
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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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In addition to the above, shelve supports all methods that are
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supported by dictionaries. This eases the transition from dictionary
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based scripts to those requiring persistent storage.
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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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Depending on the implementation, closing a persistent dictionary may
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or may not be necessary to flush changes to disk. The \method{__del__}
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method of the \class{Shelf} class calls the \method{close} method, so the
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programmer generally need not do this explicitly.
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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{classdesc}{Shelf}{dict\optional{, binary=False}}
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A subclass of \class{UserDict.DictMixin} which stores pickled values in the
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\var{dict} object. If the \var{binary} parameter is \constant{True}, binary
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pickles will be used. This can provide much more compact storage than plain
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text pickles, depending on the nature of the objects stored in the databse.
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\end{classdesc}
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\begin{classdesc}{BsdDbShelf}{dict\optional{, binary=False}}
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A subclass of \class{Shelf} which exposes \method{first}, \method{next},
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{}\method{previous}, \method{last} and \method{set_location} which are
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available in the \module{bsddb} module but not in other database modules.
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The \var{dict} object passed to the constructor must support those methods.
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This is generally accomplished by calling one of \function{bsddb.hashopen},
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\function{bsddb.btopen} or \function{bsddb.rnopen}. The optional
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\var{binary} parameter has the same interpretation as for the \class{Shelf}
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class.
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\end{classdesc}
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\begin{classdesc}{DbfilenameShelf}{dict\optional{, flag='c'}\optional{, binary=False}}
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A subclass of \class{Shelf} which accepts a filename instead of a dict-like
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object. The underlying file will be opened using \function{anydbm.open}.
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By default, the file will be created and opened for both read and write.
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The optional \var{binary} parameter has the same interpretation as for the
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\class{Shelf} class.
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\end{classdesc}
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\begin{seealso}
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\seemodule{anydbm}{Generic interface to \code{dbm}-style databases.}
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\seemodule{bsddb}{BSD \code{db} database interface.}
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\seemodule{dbhash}{Thin layer around the \module{bsddb} which provides an
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\function{open} function like the other database modules.}
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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 \refmodule{pickle}.}
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\end{seealso}
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