dbm — Interfaces to Unix “databases”

Source code:Lib/dbm/__init__.py

dbm is a generic interface to variants of the DBM database:

If none of these modules are installed, theslow-but-simple implementation in moduledbm.dumb will be used. Thereis athird party interface tothe Oracle Berkeley DB.

exceptiondbm.error

A tuple containing the exceptions that can be raised by each of the supportedmodules, with a unique exception also nameddbm.error as the firstitem — the latter is used whendbm.error is raised.

dbm.whichdb(filename)

This function attempts to guess which of the several simple database modulesavailable —dbm.sqlite3,dbm.gnu,dbm.ndbm,ordbm.dumb — should be used to open a given file.

Return one of the following values:

  • None if the file can’t be opened because it’s unreadable or doesn’t exist

  • the empty string ('') if the file’s format can’t be guessed

  • a string containing the required module name, such as'dbm.ndbm' or'dbm.gnu'

Changed in version 3.11:filename accepts apath-like object.

dbm.open(file,flag='r',mode=0o666)

Open a database and return the corresponding database object.

Parameters:

  • file (path-like object) –

    The database file to open.

    If the database file already exists, thewhichdb() function is used todetermine its type and the appropriate module is used; if it does not exist,the first submodule listed above that can be imported is used.

  • flag (str) –

    • 'r' (default): Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c': Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

  • mode (int) – The Unix file access mode of the file (default: octal0o666),used only when the database has to be created.

Changed in version 3.11:file accepts apath-like object.

The object returned byopen() supports the same basic functionality as adict; keys and their corresponding values can be stored, retrieved, anddeleted, and thein operator and thekeys() method areavailable, as well asget() andsetdefault() methods.

Key and values are always stored asbytes. This means that whenstrings are used they are implicitly converted to the default encoding beforebeing stored.

These objects also support being used in awith statement, whichwill automatically close them when done.

Changed in version 3.2:get() andsetdefault() methods are now available for alldbm backends.

Changed in version 3.4:Added native support for the context management protocol to the objectsreturned byopen().

Changed in version 3.8:Deleting a key from a read-only database raises a database module specific exceptioninstead ofKeyError.

The following example records some hostnames and a corresponding title, andthen prints out the contents of the database:

importdbm# Open database, creating it if necessary.withdbm.open('cache','c')asdb:# Record some valuesdb[b'hello']=b'there'db['www.python.org']='Python Website'db['www.cnn.com']='Cable News Network'# Note that the keys are considered bytes now.assertdb[b'www.python.org']==b'Python Website'# Notice how the value is now in bytes.assertdb['www.cnn.com']==b'Cable News Network'# Often-used methods of the dict interface work too.print(db.get('python.org',b'not present'))# Storing a non-string key or value will raise an exception (most# likely a TypeError).db['www.yahoo.com']=4# db is automatically closed when leaving the with statement.

See also

Moduleshelve

Persistence module which stores non-string data.

The individual submodules are described in the following sections.

dbm.sqlite3 — SQLite backend for dbm

Added in version 3.13.

Source code:Lib/dbm/sqlite3.py

This module uses the standard librarysqlite3 module to provide anSQLite backend for thedbm module.The files created bydbm.sqlite3 can thus be opened bysqlite3,or any other SQLite browser, including the SQLite CLI.

Availability: not WASI.

This module does not work or is not available on WebAssembly. SeeWebAssembly platforms for more information.

dbm.sqlite3.open(filename,/,flag='r',mode=0o666)

Open an SQLite database.The returned object behaves like amapping,implements aclose() method,and supports a “closing” context manager via thewith keyword.

Parameters:

  • filename (path-like object) – The path to the database to be opened.

  • flag (str) –

    • 'r' (default): Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c': Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

  • mode – The Unix file access mode of the file (default: octal0o666),used only when the database has to be created.

dbm.gnu — GNU database manager

Source code:Lib/dbm/gnu.py

Thedbm.gnu module provides an interface to theGDBMlibrary, similar to thedbm.ndbm module, but with additionalfunctionality like crash tolerance.

Note

The file formats created bydbm.gnu anddbm.ndbm are incompatibleand can not be used interchangeably.

Availability: not Android, not iOS, not WASI.

This module is not supported onmobile platformsorWebAssembly platforms.

exceptiondbm.gnu.error

Raised ondbm.gnu-specific errors, such as I/O errors.KeyError israised for general mapping errors like specifying an incorrect key.

dbm.gnu.open(filename,flag='r',mode=0o666,/)

Open a GDBM database and return agdbm object.

Parameters:

  • filename (path-like object) – The database file to open.

  • flag (str) –

    • 'r' (default): Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c': Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

    The following additional characters may be appendedto control how the database is opened:

    • 'f': Open the database in fast mode.Writes to the database will not be synchronized.

    • 's': Synchronized mode.Changes to the database will be written immediately to the file.

    • 'u': Do not lock database.

    Not all flags are valid for all versions of GDBM.See theopen_flags member for a list of supported flag characters.

  • mode (int) – The Unix file access mode of the file (default: octal0o666),used only when the database has to be created.

Raises:

error – If an invalidflag argument is passed.

Changed in version 3.11:filename accepts apath-like object.

dbm.gnu.open_flags

A string of characters theflag parameter ofopen() supports.

gdbm objects behave similar tomappings,butitems() andvalues() methods are not supported.The following methods are also provided:

gdbm.firstkey()

It’s possible to loop over every key in the database using this method and thenextkey() method. The traversal is ordered by GDBM’s internalhash values, and won’t be sorted by the key values. This method returnsthe starting key.

gdbm.nextkey(key)

Returns the key that followskey in the traversal. The following code printsevery key in the databasedb, without having to create a list in memory thatcontains them all:

k=db.firstkey()whilekisnotNone:print(k)k=db.nextkey(k)
gdbm.reorganize()

If you have carried out a lot of deletions and would like to shrink the spaceused by the GDBM file, this routine will reorganize the database.gdbmobjects will not shorten the length of a database file except by using thisreorganization; otherwise, deleted file space will be kept and reused as new(key, value) pairs are added.

gdbm.sync()

When the database has been opened in fast mode, this method forces anyunwritten data to be written to the disk.

gdbm.close()

Close the GDBM database.

gdbm.clear()

Remove all items from the GDBM database.

Added in version 3.13.

dbm.ndbm — New Database Manager

Source code:Lib/dbm/ndbm.py

Thedbm.ndbm module provides an interface to theNDBM library.This module can be used with the “classic” NDBM interface or theGDBM compatibility interface.

Note

The file formats created bydbm.gnu anddbm.ndbm are incompatibleand can not be used interchangeably.

Warning

The NDBM library shipped as part of macOS has an undocumented limitation on thesize of values, which can result in corrupted database fileswhen storing values larger than this limit. Reading such corrupted files canresult in a hard crash (segmentation fault).

Availability: not Android, not iOS, not WASI.

This module is not supported onmobile platformsorWebAssembly platforms.

exceptiondbm.ndbm.error

Raised ondbm.ndbm-specific errors, such as I/O errors.KeyError is raisedfor general mapping errors like specifying an incorrect key.

dbm.ndbm.library

Name of the NDBM implementation library used.

dbm.ndbm.open(filename,flag='r',mode=0o666,/)

Open an NDBM database and return anndbm object.

Parameters:

  • filename (path-like object) – The basename of the database file(without the.dir or.pag extensions).

  • flag (str) –

    • 'r' (default): Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c': Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

  • mode (int) – The Unix file access mode of the file (default: octal0o666),used only when the database has to be created.

ndbm objects behave similar tomappings,butitems() andvalues() methods are not supported.The following methods are also provided:

Changed in version 3.11:Acceptspath-like object for filename.

ndbm.close()

Close the NDBM database.

ndbm.clear()

Remove all items from the NDBM database.

Added in version 3.13.

dbm.dumb — Portable DBM implementation

Source code:Lib/dbm/dumb.py

Note

Thedbm.dumb module is intended as a last resort fallback for thedbm module when a more robust module is not available. Thedbm.dumbmodule is not written for speed and is not nearly as heavily used as the otherdatabase modules.

Thedbm.dumb module provides a persistentdict-likeinterface which is written entirely in Python.Unlike otherdbm backends, such asdbm.gnu, noexternal library is required.

Thedbm.dumb module defines the following:

exceptiondbm.dumb.error

Raised ondbm.dumb-specific errors, such as I/O errors.KeyError israised for general mapping errors like specifying an incorrect key.

dbm.dumb.open(filename,flag='c',mode=0o666)

Open adbm.dumb database.The returned database object behaves similar to amapping,in addition to providingsync() andclose()methods.

Parameters:

  • filename –

    The basename of the database file (without extensions).A new database creates the following files:

    • filename.dat

    • filename.dir

  • flag (str) –

    • 'r': Open existing database for reading only.

    • 'w': Open existing database for reading and writing.

    • 'c' (default): Open database for reading and writing, creating it if it doesn’t exist.

    • 'n': Always create a new, empty database, open for reading and writing.

  • mode (int) – The Unix file access mode of the file (default: octal0o666),used only when the database has to be created.

Warning

It is possible to crash the Python interpreter when loading a databasewith a sufficiently large/complex entry due to stack depth limitations inPython’s AST compiler.

Changed in version 3.5:open() always creates a new database whenflag is'n'.

Changed in version 3.8:A database opened read-only ifflag is'r'.A database is not created if it does not exist ifflag is'r' or'w'.

Changed in version 3.11:filename accepts apath-like object.

In addition to the methods provided by thecollections.abc.MutableMapping class,the following methods are provided:

dumbdbm.sync()

Synchronize the on-disk directory and data files. This method is calledby theshelve.Shelf.sync() method.

dumbdbm.close()

Close the database.