Source code:
———
dbm is a generic interface to variants of the DBM database:
If none of these modules are installed, the slow-but-simple implementation in module
will be used. There is a
to the Oracle Berkeley DB.
exceptiondbm.error
A tuple containing the exceptions that can be raised by each of the supported modules, with a unique exception also named
as the first item — the latter is used when dbm.error is raised.
dbm.whichdb(filename)
This function attempts to guess which of the several simple database modules available —
,
,
, or
— 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'
dbm.open(file, flag='r', mode=0o666)
Open a database and return the corresponding database object.
Parameters:file (
) –
The database file to open.
If the database file already exists, the
function is used to determine 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 (
) –
'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: octal 0o666), used only when the database has to be created.
The object returned by
supports the basic functionality of mutable
; keys and their corresponding values can be stored, retrieved, and deleted, and iteration, the
operator and methods keys(), get(), setdefault() and clear() are available. The keys() method returns a list instead of a view object. The setdefault() method requires two arguments.
Key and values are always stored as
. 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
statement, which will automatically close them when done.
Changed in version 3.2: get() and setdefault() methods are now available for all dbm backends.
Changed in version 3.4: Added native support for the context management protocol to the objects returned by
.
Changed in version 3.8: Deleting a key from a read-only database raises a database module specific exception instead of
.
Changed in version 3.13: clear() methods are now available for all dbm backends.
The following example records some hostnames and a corresponding title, and then 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
Module
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:
———
This module uses the standard library
module to provide an SQLite backend for the dbm module. The files created by dbm.sqlite3 can thus be opened by sqlite3, or any other SQLite browser, including the SQLite CLI.
dbm.sqlite3.open(filename, /, flag='r', mode=0o666)
Open an SQLite database.
Parameters:filename (
) – The path to the database to be opened.
flag (
) –
'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: octal 0o666), used only when the database has to be created.
The returned database object behaves similar to a mutable
, but the keys() method returns a list, and the setdefault() method requires two arguments. It also supports a “closing” context manager via the
keyword.
The following method is also provided:
sqlite3.close()
Close the SQLite database.
dbm.gnu — GNU database manager
Source code:
———
The dbm.gnu module provides an interface to the GDBM library, similar to the
module, but with additional functionality like crash tolerance.
Note
The file formats created by dbm.gnu and
are incompatible and can not be used interchangeably.
exceptiondbm.gnu.error
Raised on dbm.gnu-specific errors, such as I/O errors.
is raised for general mapping errors like specifying an incorrect key.
dbm.gnu.open_flags
A string of characters the flag parameter of
supports.
dbm.gnu.open(filename, flag='r', mode=0o666, /)
Open a GDBM database and return a gdbm object.
Parameters:filename (
) – The database file to open.
flag (
) –
'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 appended to 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 the
member for a list of supported flag characters.
mode (
) – The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.
Raises:
– If an invalid flag argument is passed.
gdbm objects behave similar to mutable
, but methods items(), values(), pop(), popitem(), and update() are not supported, the keys() method returns a list, and the setdefault() method requires two arguments. It also supports a “closing” context manager via the
keyword.
Changed in version 3.2: Added the get() and setdefault() methods.
Changed in version 3.13: Added the clear() method.
The following methods are also provided:
gdbm.close()
Close the GDBM database.
gdbm.firstkey()
It’s possible to loop over every key in the database using this method and the
method. The traversal is ordered by GDBM’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:
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 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.
dbm.ndbm — New Database Manager
Source code:
———
The dbm.ndbm module provides an interface to the NDBM library. This module can be used with the “classic” NDBM interface or the GDBM compatibility interface.
Note
The file formats created by
and dbm.ndbm are incompatible and can not be used interchangeably.
Warning
The NDBM library shipped as part of macOS has an undocumented limitation on the size of values, which can result in corrupted database files when storing values larger than this limit. Reading such corrupted files can result in a hard crash (segmentation fault).
exceptiondbm.ndbm.error
Raised on dbm.ndbm-specific errors, such as I/O errors.
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='r', mode=0o666, /)
Open an NDBM database and return an ndbm object.
Parameters:filename (
) – The basename of the database file (without the .dir or .pag extensions).
flag (
) –
'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: octal 0o666), used only when the database has to be created.
ndbm objects behave similar to mutable
, but methods items(), values(), pop(), popitem(), and update() are not supported, the keys() method returns a list, and the setdefault() method requires two arguments. It also supports a “closing” context manager via the
keyword.
Changed in version 3.2: Added the get() and setdefault() methods.
Changed in version 3.13: Added the clear() method.
The following method is also provided:
ndbm.close()
Close the NDBM database.
dbm.dumb — Portable DBM implementation
Source code:
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
-like interface which is written entirely in Python. Unlike other dbm backends, such as
, no external library is required.
The dbm.dumb module defines the following:
exceptiondbm.dumb.error
Raised on dbm.dumb-specific errors, such as I/O errors.
is raised for general mapping errors like specifying an incorrect key.
dbm.dumb.open(filename, flag='c', mode=0o666)
Open a dbm.dumb database.
Parameters:filename –
The basename of the database file (without extensions). A new database creates the following files:
filename.dat
filename.dir
flag (
) –
'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 (
) – The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.
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 flag is 'n'.
Changed in version 3.8: A database opened read-only if flag is 'r'. A database is not created if it does not exist if flag is 'r' or 'w'.
The returned database object behaves similar to a mutable
, but the keys() and items() methods return lists, and the setdefault() method requires two arguments. It also supports a “closing” context manager via the
keyword.
The following methods are also provided:
dumbdbm.close()
Close the database.
dumbdbm.sync()
Synchronize the on-disk directory and data files. This method is called by the
method.