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author Guido van Rossum <guido@python.org>
date Thu, 17 Jul 1997 16:29:42 +0000
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1 \section{Built-in Module \sectcode{gdbm}}
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2 \label{module-gdbm}
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3 \bimodindex{gdbm}
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4
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5 This module is quite similar to the \code{dbm} module, but uses {\sc gdbm}
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6 instead to provide some additional functionality. Please note that
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7 the file formats created by {\sc gdbm} and dbm are incompatible.
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8 \bimodindex{dbm}
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9
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10 The \code{gdbm} module provides an interface to the GNU DBM
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11 library. {\sc gdbm} objects behave like mappings
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12 (dictionaries), except that keys and values are always strings.
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13 Printing a {\sc gdbm} object doesn't print the keys and values, and the
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14 \code{items()} and \code{values()} methods are not supported.
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15
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16 The module defines the following constant and functions:
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17
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18 \renewcommand{\indexsubitem}{(in module dbm)}
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19 \begin{excdesc}{error}
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20 Raised on dbm-specific errors, such as I/O errors. \code{KeyError} is
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21 raised for general mapping errors like specifying an incorrect key.
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22 \end{excdesc}
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23
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24 \begin{funcdesc}{open}{filename\, \optional{flag\, \optional{mode}}}
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25 Open a dbm database and return a dbm object. The \var{filename}
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26 argument is the name of the database file (without the \file{.dir} or
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27 \file{.pag} extensions).
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28
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29 The optional \var{flag} argument can be
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30 \code{'r'} (to open an existing database for reading only --- default),
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31 \code{'w'} (to open an existing database for reading and writing),
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32 \code{'c'} (which creates the database if it doesn't exist), or
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33 \code{'n'} (which always creates a new empty database).
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34
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35 Appending \code{f} to the flag opens the database in fast mode;
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36 altered data will not automatically be written to the disk after every
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37 change. This results in faster writes to the database, but may result
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38 in an inconsistent database if the program crashes while the database
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39 is still open. Use the \code{sync()} method to force any unwritten
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40 data to be written to the disk.
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41
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42 The optional \var{mode} argument is the \UNIX{} mode of the file, used
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43 only when the database has to be created. It defaults to octal
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44 \code{0666}.
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45 \end{funcdesc}
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46
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47 In addition to the dictionary-like methods, {\sc gdbm} objects have the
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48 following methods:
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49
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50 \begin{funcdesc}{firstkey}{}
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51 It's possible to loop over every key in the database using this method
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52 and the \code{nextkey()} method. The traversal is ordered by {\sc gdbm}'s
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53 internal hash values, and won't be sorted by the key values. This
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54 method returns the starting key.
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55 \end{funcdesc}
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56
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57 \begin{funcdesc}{nextkey}{key}
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58 Returns the key that follows \var{key} in the traversal. The
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59 following code prints every key in the database \code{db}, without having to
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60 create a list in memory that contains them all:
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61 \bcode\begin{verbatim}
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62 k=db.firstkey()
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63 while k!=None:
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64 print k
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65 k=db.nextkey(k)
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66 \end{verbatim}\ecode
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67 \end{funcdesc}
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68
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69 \begin{funcdesc}{reorganize}{}
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70 If you have carried out a lot of deletions and would like to shrink
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71 the space used by the {\sc gdbm} file, this routine will reorganize the
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72 database. {\sc gdbm} will not shorten the length of a database file except
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73 by using this reorganization; otherwise, deleted file space will be
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74 kept and reused as new (key,value) pairs are added.
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75 \end{funcdesc}
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76
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77 \begin{funcdesc}{sync}{}
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78 When the database has been opened in fast mode, this method forces any
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79 unwritten data to be written to the disk.
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80 \end{funcdesc}
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81