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81 lines
3.2 KiB
TeX
81 lines
3.2 KiB
TeX
\section{Built-in Module \sectcode{gdbm}}
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\label{module-gdbm}
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\bimodindex{gdbm}
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This module is quite similar to the \code{dbm} module, but uses {\sc gdbm}
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instead to provide some additional functionality. Please note that
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the file formats created by {\sc gdbm} and dbm are incompatible.
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\bimodindex{dbm}
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The \code{gdbm} module provides an interface to the GNU DBM
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library. {\sc gdbm} objects behave like mappings
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(dictionaries), except that keys and values are always strings.
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Printing a {\sc gdbm} object doesn't print the keys and values, and the
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\code{items()} and \code{values()} methods are not supported.
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The module defines the following constant and functions:
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\renewcommand{\indexsubitem}{(in module dbm)}
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\begin{excdesc}{error}
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Raised on dbm-specific errors, such as I/O errors. \code{KeyError} is
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raised for general mapping errors like specifying an incorrect key.
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\end{excdesc}
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\begin{funcdesc}{open}{filename\, \optional{flag\, \optional{mode}}}
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Open a dbm database and return a dbm object. The \var{filename}
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argument is the name of the database file (without the \file{.dir} or
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\file{.pag} extensions).
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The optional \var{flag} argument can be
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\code{'r'} (to open an existing database for reading only --- default),
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\code{'w'} (to open an existing database for reading and writing),
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\code{'c'} (which creates the database if it doesn't exist), or
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\code{'n'} (which always creates a new empty database).
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Appending \code{f} to the flag opens the database in fast mode;
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altered data will not automatically be written to the disk after every
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change. This results in faster writes to the database, but may result
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in an inconsistent database if the program crashes while the database
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is still open. Use the \code{sync()} method to force any unwritten
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data to be written to the disk.
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The optional \var{mode} argument is the \UNIX{} mode of the file, used
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only when the database has to be created. It defaults to octal
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\code{0666}.
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\end{funcdesc}
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In addition to the dictionary-like methods, {\sc gdbm} objects have the
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following methods:
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\begin{funcdesc}{firstkey}{}
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It's possible to loop over every key in the database using this method
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and the \code{nextkey()} method. The traversal is ordered by {\sc gdbm}'s
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internal hash values, and won't be sorted by the key values. This
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method returns the starting key.
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\end{funcdesc}
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\begin{funcdesc}{nextkey}{key}
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Returns the key that follows \var{key} in the traversal. The
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following code prints every key in the database \code{db}, without having to
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create a list in memory that contains them all:
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\bcode\begin{verbatim}
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k=db.firstkey()
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while k!=None:
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print k
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k=db.nextkey(k)
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\end{verbatim}\ecode
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\end{funcdesc}
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\begin{funcdesc}{reorganize}{}
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If you have carried out a lot of deletions and would like to shrink
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the space used by the {\sc gdbm} file, this routine will reorganize the
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database. {\sc gdbm} will not shorten the length of a database file except
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by using this reorganization; otherwise, deleted file space will be
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kept and reused as new (key,value) pairs are added.
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\end{funcdesc}
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\begin{funcdesc}{sync}{}
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When the database has been opened in fast mode, this method forces any
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unwritten data to be written to the disk.
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\end{funcdesc}
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