gdbm — GNU’s reinterpretation of dbm
Platforms: Unix
Note
The gdbm module has been renamed to dbm.gnu in Python 3.0. The
2to3 tool will automatically adapt imports when converting your
sources to 3.0.
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 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 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 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 != 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.
-
gdbm.sync()
- When the database has been opened in fast mode, this method forces any
unwritten data to be written to the disk.
See also
- Module anydbm
- Generic interface to dbm-style databases.
- Module whichdb
- Utility module used to determine the type of an existing database.