dbm — Interfaces to Unix “databases”
Source code: Lib/dbm/__init__.py
dbm
is a generic interface to variants of the DBM database — dbm.gnu
or dbm.ndbm
. If none of these modules is installed, the slow-but-simple implementation in module dbm.dumb
will be used. There is a third party interface to the Oracle Berkeley DB.
-
exception dbm.error
-
A tuple containing the exceptions that can be raised by each of the supported modules, with a unique exception also named
dbm.error
as the first item — the latter is used whendbm.error
is raised.
-
dbm.whichdb(filename)
-
This function attempts to guess which of the several simple database modules available —
dbm.gnu
,dbm.ndbm
ordbm.dumb
— should be used to open a given file.Returns 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; or a string containing the required module name, such as'dbm.ndbm'
or'dbm.gnu'
.
-
dbm.open(file, flag='r', mode=0o666)
-
Open the database file file and return a corresponding object.
If the database file already exists, the
whichdb()
function is used to determine its type and the appropriate module is used; if it does not exist, the first module listed above that can be imported is used.The optional flag argument can be:
Value
Meaning
'r'
Open existing database for reading only (default)
'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 optional mode argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal
0o666
(and will be modified by the prevailing umask).
The object returned by open()
supports the same basic functionality as dictionaries; keys and their corresponding values can be stored, retrieved, and deleted, and the in
operator and the keys()
method are available, as well as get()
and setdefault()
.
Changed in version 3.2: get()
and setdefault()
are now available in all database modules.
Changed in version 3.8: Deleting a key from a read-only database raises database module specific error instead of KeyError
.
Key and values are always stored as bytes. This means that when strings are used they are implicitly converted to the default encoding before being stored.
These objects also support being used in a with
statement, which will automatically close them when done.
Changed in version 3.4: Added native support for the context management protocol to the objects returned by open()
.
The following example records some hostnames and a corresponding title, and then prints out the contents of the database:
import dbm # Open database, creating it if necessary. with dbm.open('cache', 'c') as db: # Record some values db[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. assert db[b'www.python.org'] == b'Python Website' # Notice how the value is now in bytes. assert db['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
-
Module
shelve
-
Persistence module which stores non-string data.
The individual submodules are described in the following sections.
dbm.gnu — GNU’s reinterpretation of dbm
Source code: Lib/dbm/gnu.py
This module is quite similar to the dbm
module, but uses the GNU library gdbm
instead to provide some additional functionality. Please note that the file formats created by dbm.gnu
and dbm.ndbm
are incompatible.
The dbm.gnu
module provides an interface to the GNU DBM library. dbm.gnu.gdbm
objects behave like mappings (dictionaries), except that keys and values are always converted to bytes before storing. Printing a gdbm
object doesn’t print the keys and values, and the items()
and values()
methods are not supported.
-
exception dbm.gnu.error
-
Raised on
dbm.gnu
-specific errors, such as I/O errors.KeyError
is raised for general mapping errors like specifying an incorrect key.
-
dbm.gnu.open(filename[, flag[, mode]])
-
Open a
gdbm
database and return agdbm
object. The filename argument is the name of the database file.The optional flag argument can be:
Value
Meaning
'r'
Open existing database for reading only (default)
'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 appended to the flag to control how the database is opened:
Value
Meaning
'f'
Open the database in fast mode. Writes to the database will not be synchronized.
's'
Synchronized mode. This will cause changes to the database to be immediately written to the file.
'u'
Do not lock database.
Not all flags are valid for all versions of
gdbm
. The module constantopen_flags
is a string of supported flag characters. The exceptionerror
is raised if an invalid flag is specified.The optional mode argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal
0o666
.In addition to the dictionary-like methods,
gdbm
objects have the following methods:-
gdbm.firstkey()
-
It’s possible to loop over every key in the database using this method and the
nextkey()
method. The traversal is ordered bygdbm
’s internal hash values, and won’t be sorted by the key values. This method returns the starting key.
-
gdbm.nextkey(key)
-
Returns the key that follows key in the traversal. The following code prints every key in the database
db
, without having to create a list in memory that contains them all:PythonCopy Codek = db.firstkey() while k != None: print(k) k = db.nextkey(k)
-
gdbm.reorganize()
-
If you have carried out a lot of deletions and would like to shrink the space used by the
gdbm
file, this routine will reorganize the database.gdbm
objects will not shorten the length of a database file except by using this reorganization; 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 any unwritten data to be written to the disk.
-
gdbm.close()
-
Close the
gdbm
database.
-
dbm.ndbm — Interface based on ndbm
Source code: Lib/dbm/ndbm.py
The dbm.ndbm
module provides an interface to the Unix “(n)dbm” library. Dbm objects behave like mappings (dictionaries), except that keys and values are always stored as bytes. Printing a dbm
object doesn’t print the keys and values, and the items()
and values()
methods are not supported.
This module can be used with the “classic” ndbm interface or the GNU GDBM compatibility interface. On Unix, the configure script will attempt to locate the appropriate header file to simplify building this module.
-
exception dbm.ndbm.error
-
Raised on
dbm.ndbm
-specific errors, such as I/O errors.KeyError
is raised for general mapping errors like specifying an incorrect key.
-
dbm.ndbm.library
-
Name of the
ndbm
implementation library used.
-
dbm.ndbm.open(filename[, flag[, mode]])
-
Open a dbm database and return a
ndbm
object. The filename argument is the name of the database file (without the.dir
or.pag
extensions).The optional flag argument must be one of these values:
Value
Meaning
'r'
Open existing database for reading only (default)
'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 optional mode argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal
0o666
(and will be modified by the prevailing umask).In addition to the dictionary-like methods,
ndbm
objects provide the following method:-
ndbm.close()
-
Close the
ndbm
database.
-
dbm.dumb — Portable DBM implementation
Source code: Lib/dbm/dumb.py
Note
The dbm.dumb
module is intended as a last resort fallback for the dbm
module when a more robust module is not available. The dbm.dumb
module is not written for speed and is not nearly as heavily used as the other database modules.
The dbm.dumb
module provides a persistent dictionary-like interface which is written entirely in Python. Unlike other modules such as dbm.gnu
no external library is required. As with other persistent mappings, the keys and values are always stored as bytes.
The module defines the following:
-
exception dbm.dumb.error
-
Raised on
dbm.dumb
-specific errors, such as I/O errors.KeyError
is raised for general mapping errors like specifying an incorrect key.
-
dbm.dumb.open(filename[, flag[, mode]])
-
Open a
dumbdbm
database and return a dumbdbm object. The filename argument is the basename of the database file (without any specific extensions). When a dumbdbm database is created, files with.dat
and.dir
extensions are created.The optional flag argument can be:
Value
Meaning
'r'
Open existing database for reading only (default)
'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 optional mode argument is the Unix mode of the file, used only when the database has to be created. It defaults to octal
0o666
(and will be modified by the prevailing umask).Warning
It is possible to crash the Python interpreter when loading a database with a sufficiently large/complex entry due to stack depth limitations in Python’s AST compiler.
Changed in version 3.5:
open()
always creates a new database when the flag has the value'n'
.Changed in version 3.8: A database opened with flags
'r'
is now read-only. Opening with flags'r'
and'w'
no longer creates a database if it does not exist.In addition to the methods provided by the
collections.abc.MutableMapping
class,dumbdbm
objects provide the following methods:-
dumbdbm.sync()
-
Synchronize the on-disk directory and data files. This method is called by the
Shelve.sync()
method.
-
dumbdbm.close()
-
Close the
dumbdbm
database.
-
License
© 2001–2021 Python Software Foundation
Licensed under the PSF License.
https://docs.python.org/3.9/library/dbm.html