Doc/lib/libgdbm.tex

   1 \section{Built-in Module \module{gdbm}}
   2 \label{module-gdbm}
   3 \bimodindex{gdbm}
   4
   5 % Note that if this section appears on the same page as the first
   6 % paragraph of the dbm module section, makeindex will produce the
   7 % warning:
   8 %
   9 % ## Warning (input = lib.idx, line = 1184; output = lib.ind, line = 852):
  10 %    -- Conflicting entries: multiple encaps for the same page under same key.
  11 %
  12 % This is because the \bimodindex{gdbm} and \refbimodindex{gdbm}
  13 % entries in the .idx file are slightly different (the \bimodindex{}
  14 % version includes "|textbf" at the end to make the defining occurance
  15 % bold).  There doesn't appear to be anything that can be done about
  16 % this; it's just a little annoying.  The warning can be ignored, but
  17 % the index produced uses the non-bold version.
  18
  19 This module is quite similar to the \code{dbm} module, but uses \code{gdbm}
  20 instead to provide some additional functionality.  Please note that
  21 the file formats created by \code{gdbm} and \code{dbm} are incompatible.
  22 \refbimodindex{dbm}
  23
  24 The \code{gdbm} module provides an interface to the GNU DBM
  25 library.  \code{gdbm} objects behave like mappings
  26 (dictionaries), except that keys and values are always strings.
  27 Printing a \code{gdbm} object doesn't print the keys and values, and the
  28 \code{items()} and \code{values()} methods are not supported.
  29
  30 The module defines the following constant and functions:
  31
  32 \begin{excdesc}{error}
  33 Raised on \code{gdbm}-specific errors, such as I/O errors. \code{KeyError} is
  34 raised for general mapping errors like specifying an incorrect key.
  35 \end{excdesc}
  36
  37 \begin{funcdesc}{open}{filename, \optional{flag, \optional{mode}}}
  38 Open a \code{gdbm} database and return a \code{gdbm} object.  The
  39 \var{filename} argument is the name of the database file.
  40
  41 The optional \var{flag} argument can be
  42 \code{'r'} (to open an existing database for reading only --- default),
  43 \code{'w'} (to open an existing database for reading and writing),
  44 \code{'c'} (which creates the database if it doesn't exist), or
  45 \code{'n'} (which always creates a new empty database).
  46
  47 Appending \code{f} to the flag opens the database in fast mode;
  48 altered data will not automatically be written to the disk after every
  49 change.  This results in faster writes to the database, but may result
  50 in an inconsistent database if the program crashes while the database
  51 is still open.  Use the \code{sync()} method to force any unwritten
  52 data to be written to the disk.
  53
  54 The optional \var{mode} argument is the \UNIX{} mode of the file, used
  55 only when the database has to be created.  It defaults to octal
  56 \code{0666}.
  57 \end{funcdesc}
  58
  59 In addition to the dictionary-like methods, \code{gdbm} objects have the
  60 following methods:
  61
  62 \begin{funcdesc}{firstkey}{}
  63 It's possible to loop over every key in the database using this method
  64 and the \code{nextkey()} method.  The traversal is ordered by \code{gdbm}'s
  65 internal hash values, and won't be sorted by the key values.  This
  66 method returns the starting key.
  67 \end{funcdesc}
  68
  69 \begin{funcdesc}{nextkey}{key}
  70 Returns the key that follows \var{key} in the traversal.  The
  71 following code prints every key in the database \code{db}, without having to
  72 create a list in memory that contains them all:
  73 \begin{verbatim}
  74 k=db.firstkey()
  75 while k!=None:
  76     print k
  77     k=db.nextkey(k)
  78 \end{verbatim}
  79 \end{funcdesc}
  80
  81 \begin{funcdesc}{reorganize}{}
  82 If you have carried out a lot of deletions and would like to shrink
  83 the space used by the \code{gdbm} file, this routine will reorganize the
  84 database.  \code{gdbm} will not shorten the length of a database file except
  85 by using this reorganization; otherwise, deleted file space will be
  86 kept and reused as new (key,value) pairs are added.
  87 \end{funcdesc}
  88
  89 \begin{funcdesc}{sync}{}
  90 When the database has been opened in fast mode, this method forces any
  91 unwritten data to be written to the disk.
  92 \end{funcdesc}
  93