This functionattempts to guess which of the several simple database modulesavailable —dbm.bsd,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 openedbecause it’s unreadable or doesn’t exist; the empty string ('') if thefile’s format can’t be guessed; or a string containing the required modulename, such as'dbm.ndbm' or'dbm.gnu'.
Open the database filefilename and return a corresponding object.
If the database file already exists, thewhichdb() function is used todetermine 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 optionalflag argument can be'r' to open an existing database forreading only,'w' to open an existing database for reading and writing,'c' to create the database if it doesn’t exist, or'n', which willalways create a new empty database. If not specified, the default value is'r'.
The optionalmode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal0o666 (and will bemodified by the prevailing umask).
The object returned byopen() supports most of the same functionality asdictionaries; keys and their corresponding values can be stored, retrieved, anddeleted, and thein operator and thekeys() method areavailable. Key and values are always stored as bytes. This means that whenstrings are used they are implicitly converted to the default encoding beforebeing stored.
The following example records some hostnames and a corresponding title, andthen prints out the contents of the database:
importdbm# Open database, creating it if necessary.db=dbm.open('cache','c')# 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'# Loop through contents. Other dictionary methods# such as .keys(), .values() also work.fork,vindb.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 thedbm module, but uses the GNU librarygdbm instead to provide some additional functionality. Please note that thefile formats created bydbm.gnu anddbm.ndbm are incompatible.
Thedbm.gnu module provides an interface to the GNU DBM library.dbm.gnu.gdbm objects behave like mappings (dictionaries), except that keys andvalues are always converted to bytes before storing. Printing agdbmobject doesn’t print thekeys and values, and theitems() andvalues() methods are notsupported.
Open agdbm database and return agdbm object. Thefilenameargument is the name of the database file.
The optionalflag argument can be:
| Value | Meaning |
|---|---|
| 'r' | Open existing database for reading only(default) |
| 'w' | Open existing database for reading andwriting |
| 'c' | Open database for reading and writing,creating it if it doesn’t exist |
| 'n' | Always create a new, empty database, openfor reading and writing |
The following additional characters may be appended to the flag to controlhow the database is opened:
| Value | Meaning |
|---|---|
| 'f' | Open the database in fast mode. Writesto the database will not be synchronized. |
| 's' | Synchronized mode. This will cause changesto the database to be immediately writtento the file. |
| 'u' | Do not lock database. |
Not all flags are valid for all versions ofgdbm. The module constantopen_flags is a string of supported flag characters. The exceptionerror is raised if an invalid flag is specified.
The optionalmode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal0o666.
In addition to the dictionary-like methods,gdbm objects have thefollowing methods:
Returns the key that followskey in the traversal. The following code printsevery key in the databasedb, without having to create a list in memory thatcontains them all:
k=db.firstkey()whilek!=None:print(k)k=db.nextkey(k)
Platforms:Unix
Thedbm.ndbm module provides an interface to the Unix “(n)dbm” library.Dbm objects behave like mappings (dictionaries), except that keys and values arealways stored as bytes. Printing adbm object doesn’t print the keys andvalues, and theitems() andvalues() methods are not supported.
This module can be used with the “classic” ndbm interface, the BSD DBcompatibility interface, or the GNU GDBM compatibility interface. On Unix, theconfigure script will attempt to locate the appropriate header fileto simplify building this module.
Open a dbm database and return adbm object. Thefilename argument is thename 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 optionalflag argument must be one of these values:
| Value | Meaning |
|---|---|
| 'r' | Open existing database for reading only(default) |
| 'w' | Open existing database for reading andwriting |
| 'c' | Open database for reading and writing,creating it if it doesn’t exist |
| 'n' | Always create a new, empty database, openfor reading and writing |
The optionalmode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal0o666 (and will bemodified by the prevailing umask).
Note
Thedbm.dumb module is intended as a last resort fallback for thedbm module when a more robust module is not available. Thedbm.dumbmodule is not written for speed and is not nearly as heavily used as the otherdatabase modules.
Thedbm.dumb module provides a persistent dictionary-like interface whichis written entirely in Python. Unlike other modules such asdbm.gnu noexternal library is required. As with other persistent mappings, the keys andvalues are always stored as bytes.
The module defines the following:
Open adumbdbm database and return a dumbdbm object. Thefilename argument isthe basename of the database file (without any specific extensions). When adumbdbm database is created, files with.dat and.dir extensionsare created.
The optionalflag argument is currently ignored; the database is always openedfor update, and will be created if it does not exist.
The optionalmode argument is the Unix mode of the file, used only when thedatabase has to be created. It defaults to octal0o666 (and will be modifiedby the prevailing umask).
In addition to the methods provided by thecollections.MutableMapping class,dumbdbm objects provide the following method:
marshal — Internal Python object serialization
sqlite3 — DB-API 2.0 interface for SQLite databases
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