Key and value types¶

The accepted types for keys and values vary by backend. Keys and values are handled the same way, except where noted below:

• Traditional backends:

  • dbm.gnu and dbm.ndbm: Accept str and bytes objects.

  • dbm.dumb: Accepts str and bytes objects; bytearray is acceptable as a value, but not as a key.

• SQLite backend (dbm.sqlite3): Accepts str, bytes, int, float, bool, bytearray, memoryview, and array.array objects.

Storage format:

All keys and values are stored as bytes objects in the database. When retrieving, you’ll always get bytes back, regardless of the original type stored.

Type conversion examples:

• db['key'] = 'string' stored as b'string'

• db['key'] = bytearray(b'data') stored as b'data' (dbm.dumb and dbm.sqlite3 only)

• db['key'] = 42 stored as b'42' (dbm.sqlite3 only)

• db['key'] = 3.14 stored as b'3.14' (dbm.sqlite3 only)

• db['key'] = True stored as b'1' (dbm.sqlite3 only)

• db['key'] = False stored as b'0' (dbm.sqlite3 only)

• db['key'] = memoryview(b'data') stored as b'data' (dbm.sqlite3 only)

• db['key'] = array.array('i', [1, 2, 3]) stored as bytes (for example, on little-endian: b'\x01\x00\x00\x00\x02\x00\x00\x00\x03\x00\x00\x00') (dbm.sqlite3 only)

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 behavior depends on the backend:
    # - Traditional backends (ndbm, gnu, dumb) will raise a TypeError
    # - The sqlite3 backend accepts it and converts to bytes
    db['www.yahoo.com'] = 4

# db is automatically closed when leaving the with statement.

The individual submodules are described in the following sections.

dbm.sqlite3 — SQLite backend for dbm¶

Source code: Lib/dbm/sqlite3.py

This module uses the standard library sqlite3 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 (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: octal 0o666), used only when the database has to be created.

The returned database object behaves similar to a mutable mapping, but the keys() method returns a list, and the setdefault() method requires two arguments. It also supports a “closing” context manager via the with keyword.

The following methods are also provided:

sqlite3.close()¶

Close the SQLite database.

sqlite3.reorganize()¶

If you have carried out a lot of deletions and would like to shrink the space used on disk, this method will reorganize the database; otherwise, deleted file space will be kept and reused as new (key, value) pairs are added.

Note

While reorganizing, as much as two times the size of the original database is required in free disk space. However, be aware that this factor changes for each dbm submodule.

Added in version 3.15.

dbm.gnu — GNU database manager¶

Source code: Lib/dbm/gnu.py

The dbm.gnu module provides an interface to the GDBM library, similar to the dbm.ndbm module, but with additional functionality like crash tolerance.

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_flags¶

A string of characters the flag parameter of open() supports.

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

Open a GDBM database and return a gdbm 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 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 open_flags member for a list of supported flag characters.

• mode (int) – The Unix file access mode of the file (default: octal 0o666), used only when the database has to be created.

Raises:

error – If an invalid flag argument is passed.

Changed in version 3.11: filename accepts a path-like object.

gdbm objects behave similar to mutable mappings, 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 with 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 nextkey() 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()
while k is not 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.

Note

While reorganizing, as much as one time the size of the original database is required in free disk space. However, be aware that this factor changes for each dbm submodule.

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: Lib/dbm/ndbm.py

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.

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='r', mode=0o666, /)¶

Open an NDBM database and return an ndbm 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: octal 0o666), used only when the database has to be created.

Changed in version 3.11: Accepts path-like object for filename.

ndbm objects behave similar to mutable mappings, 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 with 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: Lib/dbm/dumb.py

The dbm.dumb module provides a persistent dict-like interface which is written entirely in Python. Unlike other dbm backends, such as dbm.gnu, no external library is required.

The dbm.dumb 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='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 (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: 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.

Warning

dbm.dumb does not support concurrent read/write access. (Multiple simultaneous read accesses are safe.) When a program has the database open for writing, no other program should have it open for reading or writing.

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'.

Changed in version 3.11: filename accepts a path-like object.

The returned database object behaves similar to a mutable mapping, but the keys() and items() methods return lists, and the setdefault() method requires two arguments. It also supports a “closing” context manager via the with keyword.

The following methods are also provided:

dumbdbm.close()¶

Close the database.

dumbdbm.reorganize()¶

If you have carried out a lot of deletions and would like to shrink the space used on disk, this method will reorganize the database; otherwise, deleted file space will not be reused.

Note

While reorganizing, no additional free disk space is required. However, be aware that this factor changes for each dbm submodule.

Added in version 3.15.

dumbdbm.sync()¶

Synchronize the on-disk directory and data files. This method is called by the shelve.Shelf.sync() method.