This functionattempts to guess which of the several simple database modules available — dbm.bsd, dbm.gnu, dbm.ndbm or dbm.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'.
Open the database file filename 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 most of the same functionality as dictionaries; keys and their corresponding values can be stored, retrieved, and deleted, and the in operator and the keys() method are available. 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.
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.
db = dbm.open('cache', 'c')
# 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'
# Loop through contents. Other dictionary methods
# such as .keys(), .values() also work.
for k, v in db.iteritems():
print(k, '\t', v)
# Storing a non-string key or value will raise an exception (most
# likely a TypeError).
db['www.yahoo.com'] = 4
# Close when done.
db.close()
See also
The individual submodules are described in the following sections.
Platforms: Unix
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.
Open a gdbm database and return a gdbm 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 constant open_flags is a string of supported flag characters. The exception error 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:
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 != None:
print(k)
k = db.nextkey(k)
Platforms: Unix
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, the BSD DB compatibility 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.
Open a dbm database and return a dbm object. The filename argument is the name of the database file (without the .dir or .pag extensions; note that the BSD DB implementation of the interface will append the extension .db and only create one file).
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).
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:
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 is currently ignored; the database is always opened for update, and will be created if it does not exist.
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 methods provided by the collections.MutableMapping class, dumbdbm objects provide the following method: