1:mod:`dbm` --- Interfaces to Unix "databases" 2============================================= 3 4.. module:: dbm 5 :synopsis: Interfaces to various Unix "database" formats. 6 7**Source code:** :source:`Lib/dbm/__init__.py` 8 9-------------- 10 11:mod:`dbm` is a generic interface to variants of the DBM database --- 12:mod:`dbm.gnu` or :mod:`dbm.ndbm`. If none of these modules is installed, the 13slow-but-simple implementation in module :mod:`dbm.dumb` will be used. There 14is a `third party interface <https://www.jcea.es/programacion/pybsddb.htm>`_ to 15the Oracle Berkeley DB. 16 17 18.. exception:: error 19 20 A tuple containing the exceptions that can be raised by each of the supported 21 modules, with a unique exception also named :exc:`dbm.error` as the first 22 item --- the latter is used when :exc:`dbm.error` is raised. 23 24 25.. function:: whichdb(filename) 26 27 This function attempts to guess which of the several simple database modules 28 available --- :mod:`dbm.gnu`, :mod:`dbm.ndbm` or :mod:`dbm.dumb` --- should 29 be used to open a given file. 30 31 Returns one of the following values: ``None`` if the file can't be opened 32 because it's unreadable or doesn't exist; the empty string (``''``) if the 33 file's format can't be guessed; or a string containing the required module 34 name, such as ``'dbm.ndbm'`` or ``'dbm.gnu'``. 35 36 37.. function:: open(file, flag='r', mode=0o666) 38 39 Open the database file *file* and return a corresponding object. 40 41 If the database file already exists, the :func:`whichdb` function is used to 42 determine its type and the appropriate module is used; if it does not exist, 43 the first module listed above that can be imported is used. 44 45 The optional *flag* argument can be: 46 47 +---------+-------------------------------------------+ 48 | Value | Meaning | 49 +=========+===========================================+ 50 | ``'r'`` | Open existing database for reading only | 51 | | (default) | 52 +---------+-------------------------------------------+ 53 | ``'w'`` | Open existing database for reading and | 54 | | writing | 55 +---------+-------------------------------------------+ 56 | ``'c'`` | Open database for reading and writing, | 57 | | creating it if it doesn't exist | 58 +---------+-------------------------------------------+ 59 | ``'n'`` | Always create a new, empty database, open | 60 | | for reading and writing | 61 +---------+-------------------------------------------+ 62 63 The optional *mode* argument is the Unix mode of the file, used only when the 64 database has to be created. It defaults to octal ``0o666`` (and will be 65 modified by the prevailing umask). 66 67 68The object returned by :func:`.open` supports the same basic functionality as 69dictionaries; keys and their corresponding values can be stored, retrieved, and 70deleted, and the :keyword:`in` operator and the :meth:`keys` method are 71available, as well as :meth:`get` and :meth:`setdefault`. 72 73.. versionchanged:: 3.2 74 :meth:`get` and :meth:`setdefault` are now available in all database modules. 75 76Key and values are always stored as bytes. This means that when 77strings are used they are implicitly converted to the default encoding before 78being stored. 79 80These objects also support being used in a :keyword:`with` statement, which 81will automatically close them when done. 82 83.. versionchanged:: 3.4 84 Added native support for the context management protocol to the objects 85 returned by :func:`.open`. 86 87The following example records some hostnames and a corresponding title, and 88then prints out the contents of the database:: 89 90 import dbm 91 92 # Open database, creating it if necessary. 93 with dbm.open('cache', 'c') as db: 94 95 # Record some values 96 db[b'hello'] = b'there' 97 db['www.python.org'] = 'Python Website' 98 db['www.cnn.com'] = 'Cable News Network' 99 100 # Note that the keys are considered bytes now. 101 assert db[b'www.python.org'] == b'Python Website' 102 # Notice how the value is now in bytes. 103 assert db['www.cnn.com'] == b'Cable News Network' 104 105 # Often-used methods of the dict interface work too. 106 print(db.get('python.org', b'not present')) 107 108 # Storing a non-string key or value will raise an exception (most 109 # likely a TypeError). 110 db['www.yahoo.com'] = 4 111 112 # db is automatically closed when leaving the with statement. 113 114 115.. seealso:: 116 117 Module :mod:`shelve` 118 Persistence module which stores non-string data. 119 120 121The individual submodules are described in the following sections. 122 123 124:mod:`dbm.gnu` --- GNU's reinterpretation of dbm 125------------------------------------------------ 126 127.. module:: dbm.gnu 128 :platform: Unix 129 :synopsis: GNU's reinterpretation of dbm. 130 131**Source code:** :source:`Lib/dbm/gnu.py` 132 133-------------- 134 135This module is quite similar to the :mod:`dbm` module, but uses the GNU library 136``gdbm`` instead to provide some additional functionality. Please note that the 137file formats created by :mod:`dbm.gnu` and :mod:`dbm.ndbm` are incompatible. 138 139The :mod:`dbm.gnu` module provides an interface to the GNU DBM library. 140``dbm.gnu.gdbm`` objects behave like mappings (dictionaries), except that keys and 141values are always converted to bytes before storing. Printing a ``gdbm`` 142object doesn't print the 143keys and values, and the :meth:`items` and :meth:`values` methods are not 144supported. 145 146.. exception:: error 147 148 Raised on :mod:`dbm.gnu`-specific errors, such as I/O errors. :exc:`KeyError` is 149 raised for general mapping errors like specifying an incorrect key. 150 151 152.. function:: open(filename[, flag[, mode]]) 153 154 Open a ``gdbm`` database and return a :class:`gdbm` object. The *filename* 155 argument is the name of the database file. 156 157 The optional *flag* argument can be: 158 159 +---------+-------------------------------------------+ 160 | Value | Meaning | 161 +=========+===========================================+ 162 | ``'r'`` | Open existing database for reading only | 163 | | (default) | 164 +---------+-------------------------------------------+ 165 | ``'w'`` | Open existing database for reading and | 166 | | writing | 167 +---------+-------------------------------------------+ 168 | ``'c'`` | Open database for reading and writing, | 169 | | creating it if it doesn't exist | 170 +---------+-------------------------------------------+ 171 | ``'n'`` | Always create a new, empty database, open | 172 | | for reading and writing | 173 +---------+-------------------------------------------+ 174 175 The following additional characters may be appended to the flag to control 176 how the database is opened: 177 178 +---------+--------------------------------------------+ 179 | Value | Meaning | 180 +=========+============================================+ 181 | ``'f'`` | Open the database in fast mode. Writes | 182 | | to the database will not be synchronized. | 183 +---------+--------------------------------------------+ 184 | ``'s'`` | Synchronized mode. This will cause changes | 185 | | to the database to be immediately written | 186 | | to the file. | 187 +---------+--------------------------------------------+ 188 | ``'u'`` | Do not lock database. | 189 +---------+--------------------------------------------+ 190 191 Not all flags are valid for all versions of ``gdbm``. The module constant 192 :const:`open_flags` is a string of supported flag characters. The exception 193 :exc:`error` is raised if an invalid flag is specified. 194 195 The optional *mode* argument is the Unix mode of the file, used only when the 196 database has to be created. It defaults to octal ``0o666``. 197 198 In addition to the dictionary-like methods, ``gdbm`` objects have the 199 following methods: 200 201 .. method:: gdbm.firstkey() 202 203 It's possible to loop over every key in the database using this method and the 204 :meth:`nextkey` method. The traversal is ordered by ``gdbm``'s internal 205 hash values, and won't be sorted by the key values. This method returns 206 the starting key. 207 208 .. method:: gdbm.nextkey(key) 209 210 Returns the key that follows *key* in the traversal. The following code prints 211 every key in the database ``db``, without having to create a list in memory that 212 contains them all:: 213 214 k = db.firstkey() 215 while k != None: 216 print(k) 217 k = db.nextkey(k) 218 219 .. method:: gdbm.reorganize() 220 221 If you have carried out a lot of deletions and would like to shrink the space 222 used by the ``gdbm`` file, this routine will reorganize the database. ``gdbm`` 223 objects will not shorten the length of a database file except by using this 224 reorganization; otherwise, deleted file space will be kept and reused as new 225 (key, value) pairs are added. 226 227 .. method:: gdbm.sync() 228 229 When the database has been opened in fast mode, this method forces any 230 unwritten data to be written to the disk. 231 232 .. method:: gdbm.close() 233 234 Close the ``gdbm`` database. 235 236:mod:`dbm.ndbm` --- Interface based on ndbm 237------------------------------------------- 238 239.. module:: dbm.ndbm 240 :platform: Unix 241 :synopsis: The standard "database" interface, based on ndbm. 242 243**Source code:** :source:`Lib/dbm/ndbm.py` 244 245-------------- 246 247The :mod:`dbm.ndbm` module provides an interface to the Unix "(n)dbm" library. 248Dbm objects behave like mappings (dictionaries), except that keys and values are 249always stored as bytes. Printing a ``dbm`` object doesn't print the keys and 250values, and the :meth:`items` and :meth:`values` methods are not supported. 251 252This module can be used with the "classic" ndbm interface or the GNU GDBM 253compatibility interface. On Unix, the :program:`configure` script will attempt 254to locate the appropriate header file to simplify building this module. 255 256.. exception:: error 257 258 Raised on :mod:`dbm.ndbm`-specific errors, such as I/O errors. :exc:`KeyError` is raised 259 for general mapping errors like specifying an incorrect key. 260 261 262.. data:: library 263 264 Name of the ``ndbm`` implementation library used. 265 266 267.. function:: open(filename[, flag[, mode]]) 268 269 Open a dbm database and return a ``ndbm`` object. The *filename* argument is the 270 name of the database file (without the :file:`.dir` or :file:`.pag` extensions). 271 272 The optional *flag* argument must be one of these values: 273 274 +---------+-------------------------------------------+ 275 | Value | Meaning | 276 +=========+===========================================+ 277 | ``'r'`` | Open existing database for reading only | 278 | | (default) | 279 +---------+-------------------------------------------+ 280 | ``'w'`` | Open existing database for reading and | 281 | | writing | 282 +---------+-------------------------------------------+ 283 | ``'c'`` | Open database for reading and writing, | 284 | | creating it if it doesn't exist | 285 +---------+-------------------------------------------+ 286 | ``'n'`` | Always create a new, empty database, open | 287 | | for reading and writing | 288 +---------+-------------------------------------------+ 289 290 The optional *mode* argument is the Unix mode of the file, used only when the 291 database has to be created. It defaults to octal ``0o666`` (and will be 292 modified by the prevailing umask). 293 294 In addition to the dictionary-like methods, ``ndbm`` objects 295 provide the following method: 296 297 .. method:: ndbm.close() 298 299 Close the ``ndbm`` database. 300 301 302:mod:`dbm.dumb` --- Portable DBM implementation 303----------------------------------------------- 304 305.. module:: dbm.dumb 306 :synopsis: Portable implementation of the simple DBM interface. 307 308**Source code:** :source:`Lib/dbm/dumb.py` 309 310.. index:: single: databases 311 312.. note:: 313 314 The :mod:`dbm.dumb` module is intended as a last resort fallback for the 315 :mod:`dbm` module when a more robust module is not available. The :mod:`dbm.dumb` 316 module is not written for speed and is not nearly as heavily used as the other 317 database modules. 318 319-------------- 320 321The :mod:`dbm.dumb` module provides a persistent dictionary-like interface which 322is written entirely in Python. Unlike other modules such as :mod:`dbm.gnu` no 323external library is required. As with other persistent mappings, the keys and 324values are always stored as bytes. 325 326The module defines the following: 327 328 329.. exception:: error 330 331 Raised on :mod:`dbm.dumb`-specific errors, such as I/O errors. :exc:`KeyError` is 332 raised for general mapping errors like specifying an incorrect key. 333 334 335.. function:: open(filename[, flag[, mode]]) 336 337 Open a ``dumbdbm`` database and return a dumbdbm object. The *filename* argument is 338 the basename of the database file (without any specific extensions). When a 339 dumbdbm database is created, files with :file:`.dat` and :file:`.dir` extensions 340 are created. 341 342 The optional *flag* argument supports only the semantics of ``'c'`` 343 and ``'n'`` values. Other values will default to database being always 344 opened for update, and will be created if it does not exist. 345 346 The optional *mode* argument is the Unix mode of the file, used only when the 347 database has to be created. It defaults to octal ``0o666`` (and will be modified 348 by the prevailing umask). 349 350 .. warning:: 351 It is possible to crash the Python interpreter when loading a database 352 with a sufficiently large/complex entry due to stack depth limitations in 353 Python's AST compiler. 354 355 .. versionchanged:: 3.5 356 :func:`.open` always creates a new database when the flag has the value 357 ``'n'``. 358 359 .. deprecated-removed:: 3.6 3.8 360 Creating database in ``'r'`` and ``'w'`` modes. Modifying database in 361 ``'r'`` mode. 362 363 In addition to the methods provided by the 364 :class:`collections.abc.MutableMapping` class, :class:`dumbdbm` objects 365 provide the following methods: 366 367 .. method:: dumbdbm.sync() 368 369 Synchronize the on-disk directory and data files. This method is called 370 by the :meth:`Shelve.sync` method. 371 372 .. method:: dumbdbm.close() 373 374 Close the ``dumbdbm`` database. 375 376