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\section{\module{gdbm} ---
GNU's reinterpretation of dbm}
\declaremodule{builtin}{gdbm}
\platform{Unix}
\modulesynopsis{GNU's reinterpretation of dbm.}
This module is quite similar to the \refmodule{dbm}\refbimodindex{dbm}
module, but uses \code{gdbm} instead to provide some additional
functionality. Please note that the file formats created by
\code{gdbm} and \code{dbm} are incompatible.
The \module{gdbm} module provides an interface to the GNU DBM
library. \code{gdbm} objects behave like mappings
(dictionaries), except that keys and values are always strings.
Printing a \code{gdbm} object doesn't print the keys and values, and
the \method{items()} and \method{values()} methods are not supported.
The module defines the following constant and functions:
\begin{excdesc}{error}
Raised on \code{gdbm}-specific errors, such as I/O errors.
\exception{KeyError} is raised for general mapping errors like
specifying an incorrect key.
\end{excdesc}
\begin{funcdesc}{open}{filename, \optional{flag, \optional{mode}}}
Open a \code{gdbm} database and return a \code{gdbm} object. The
\var{filename} argument is the name of the database file.
The optional \var{flag} argument can be
\code{'r'} (to open an existing database for reading only --- default),
\code{'w'} (to open an existing database for reading and writing),
\code{'c'} (which creates the database if it doesn't exist), or
\code{'n'} (which always creates a new empty database).
The following additional characters may be appended to the flag to
control how the database is opened:
\begin{itemize}
\item \code{'f'} --- Open the database in fast mode. Writes to the database
will not be syncronized.
\item \code{'s'} --- Synchronized mode. This will cause changes to the database
will be immediately written to the file.
\item \code{'u'} --- Do not lock database.
\end{itemize}
Not all flags are valid for all versions of \code{gdbm}. The
module constant \code{open_flags} is a string of supported flag
characters. The exception \exception{error} is raised if an invalid
flag is specified.
The optional \var{mode} argument is the \UNIX{} mode of the file, used
only when the database has to be created. It defaults to octal
\code{0666}.
\end{funcdesc}
In addition to the dictionary-like methods, \code{gdbm} objects have the
following methods:
\begin{funcdesc}{firstkey}{}
It's possible to loop over every key in the database using this method
and the \method{nextkey()} method. The traversal is ordered by
\code{gdbm}'s internal hash values, and won't be sorted by the key
values. This method returns the starting key.
\end{funcdesc}
\begin{funcdesc}{nextkey}{key}
Returns the key that follows \var{key} in the traversal. The
following code prints every key in the database \code{db}, without
having to create a list in memory that contains them all:
\begin{verbatim}
k = db.firstkey()
while k != None:
print k
k = db.nextkey(k)
\end{verbatim}
\end{funcdesc}
\begin{funcdesc}{reorganize}{}
If you have carried out a lot of deletions and would like to shrink
the space used by the \code{gdbm} file, this routine will reorganize
the database. \code{gdbm} will not shorten the length of a database
file except by using this reorganization; otherwise, deleted file
space will be kept and reused as new (key, value) pairs are added.
\end{funcdesc}
\begin{funcdesc}{sync}{}
When the database has been opened in fast mode, this method forces any
unwritten data to be written to the disk.
\end{funcdesc}
\begin{seealso}
\seemodule{anydbm}{Generic interface to \code{dbm}-style databases.}
\seemodule{whichdb}{Utility module used to determine the type of an
existing database.}
\end{seealso}
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