# `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 when `dbm.error` is raised.

dbm.whichdb(filename)

  This function attempts to guess which of the several simple database modules available — `dbm.gnu`, `dbm.ndbm` or `dbm.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 a path-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, the `whichdb()` 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 (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: file accepts a path-like object.

The object returned by `open()` supports the same basic functionality as a `dict`; 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()` methods.

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 3.2: `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 `open()`.

Changed in version 3.8: Deleting a key from a read-only database raises a database module specific exception instead of `KeyError`.

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 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.

Note

The file formats created by `dbm.gnu` and `dbm.ndbm` are incompatible and can not be used interchangeably.

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='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.

  dbm.gnu.open_flags

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

  `gdbm` objects behave similar to mappings, but `items()` and `values()` 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 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.

  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` — 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.

Note

The file formats created by `dbm.gnu` 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).

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.

  `ndbm` objects behave similar to mappings, but `items()` and `values()` methods are not supported. The following methods are also provided:

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

  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 `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. The returned database object behaves similar to a mapping, in addition to providing `sync()` and `close()` 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: 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'`.

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

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

  dumbdbm.sync()

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

  dumbdbm.close()

    Close the database.