mirror of https://github.com/python/cpython
* document open() function
* promote the example and the documented restrictions to \subsection status * document the flag parameter of the DbfilenameShelf class
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@ -13,46 +13,35 @@ 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{funcdesc}{open}{filename\optional{,flag='c'\optional{,binary=\code{False}}}}
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Open a persistent dictionary. By default, the underlying database file is
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opened for reading and writing. The optional \var{flag} pararameter, if set
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to \code{'r'}, can be used to force the file to be opened in read-only mode.
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By default, ASCII pickles are used to serialize values. If the optional
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{}\var{binary} parameter is set to \var{True}, binary pickles will be used
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instead.
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\end{funcdesc}
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\begin{verbatim}
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import shelve
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Shelve objects support all methods supported by dictionaries. This eases
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the transition from dictionary based scripts to those requiring persistent
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storage.
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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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\subsection{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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(such as \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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(such as \refmodule{dbm}, \refmodule{gdbm} or \refmodule{bsddb}) depends on
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which interface is available. Therefore it is not safe to open the database
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directly 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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\refbimodindex{bsddb}
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\item
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Depending on the implementation, closing a persistent dictionary may
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@ -92,10 +81,35 @@ class.
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A subclass of \class{Shelf} which accepts a \var{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{flag} parameter has the same interpretation as for the
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\function{open} function.
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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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\subsection{Example}
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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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\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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