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11.9. gdbm — GNU’s reinterpretation of dbm
Note
The gdbm
module has been renamed to dbm.gnu
in Python 3. The 2to3 tool will automatically adapt imports when converting your sources to Python 3.
This module is quite similar to the dbm
module, but uses gdbm
instead to provide some additional functionality. Please note that the file formats created by gdbm
and dbm
are incompatible.
The gdbm
module provides an interface to the GNU DBM library. gdbm
objects behave like mappings (dictionaries), except that keys and values are always strings. Printing a gdbm
object doesn’t print the keys and values, and the items()
and values()
methods are not supported.
The module defines the following constant and functions:
- exception
gdbm.
error
-
Raised on
gdbm
-specific errors, such as I/O errors.KeyError
is raised for general mapping errors like specifying an incorrect key.
gdbm.
open
( filename [, flag [, mode ] ] )-
Open a
gdbm
database and return agdbm
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 constantopen_flags
is a string of supported flag characters. The exceptionerror
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
0666
.
In addition to the dictionary-like methods, gdbm
objects have the following methods:
gdbm.
firstkey
( )-
It’s possible to loop over every key in the database using this method and the
nextkey()
method. The traversal is ordered bygdbm
’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 != 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
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.