PyMOTW: anydbm (and related modules)

anydbm - Access to DBM-style databases

Purpose:	anydbm provides a generic dictionary-like interface to DBM-style, string-keyed databases
Python Version:	1.4 and later

anydbm is a front-end for DBM-style databases that use simple string values as keys to access records containing strings. It uses the whichdb module to identify dbhash, gdbm, and dbm databases, then opens them with the appropriate module. It is used as a backend for shelve, which knows how to store objects using pickle.

Creating a New Database

The storage format for new databases is selected by looking for each of these modules in order:

• dbhash


• gdbm


• dbm


• dumbdbm

The open() function takes flags to control how the database file is managed. To create a new database when necessary, use 'c'. To always create a new database, use 'n'.

import anydbm

db = anydbm.open('/tmp/example.db', 'n')
db['key'] = 'value'
db['today'] = 'Sunday'
db['author'] = 'Doug'
db.close()

In this example, the file is always re-initialized. To see what type of database was created, we can use whichdb.

import whichdb

print whichdb.whichdb('/tmp/example.db')

Your results may vary, depending on what modules are installed on your system.

$ python anydbm_whichdb.py
dbhash

Opening an Existing Database

To open an existing database, use flags of either 'r' (for read-only) or 'w' (for read-write). You don’t need to worry about the format, because existing databases are automatically given to whichdb to identify. If a file can be identified, the appropriate module is used to open it.

import anydbm

db = anydbm.open('/tmp/example.db', 'r')
try:
    print 'keys():', db.keys()
    for k, v in db.iteritems():
        print 'iterating:', k, v
    print 'db["author"] =', db['author']
finally:
    db.close()

Once open, db is a dictionary-like object, with support for the usual methods:

$ python anydbm_existing.py
keys(): ['key', 'today', 'author']
iterating: key value
iterating: today Sunday
iterating: author Doug
db["author"] = Doug

Error Cases

The keys of the database need to be strings.

import anydbm

db = anydbm.open('/tmp/example.db', 'w')
try:
    db[1] = 'one'
finally:
    db.close()

Passing another type results in a TypeError.

$ python anydbm_intkeys.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/anydbm/PyMOTW/anydbm/anydbm_intkeys.py", line 16, in <module>
    db[1] = 'one'
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/bsddb/__init__.py", line 230, in __setitem__
    _DeadlockWrap(wrapF)  # self.db[key] = value
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/bsddb/dbutils.py", line 62, in DeadlockWrap
    return function(*_args, **_kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/bsddb/__init__.py", line 229, in wrapF
    self.db[key] = value
TypeError: Integer keys only allowed for Recno and Queue DB's

Values must be strings or None.

import anydbm

db = anydbm.open('/tmp/example.db', 'w')
try:
    db['one'] = 1
finally:
    db.close()

A similar TypeError is raised if a value is not a string.

$ python anydbm_intvalue.py
Traceback (most recent call last):
  File "/Users/dhellmann/Documents/PyMOTW/in_progress/anydbm/PyMOTW/anydbm/anydbm_intvalue.py", line 16, in <module>
    db['one'] = 1
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/bsddb/__init__.py", line 230, in __setitem__
    _DeadlockWrap(wrapF)  # self.db[key] = value
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/bsddb/dbutils.py", line 62, in DeadlockWrap
    return function(*_args, **_kwargs)
  File "/Library/Frameworks/Python.framework/Versions/2.5/lib/python2.5/bsddb/__init__.py", line 229, in wrapF
    self.db[key] = value
TypeError: Data values must be of type string or None.

References

PyMOTW Home

Download Examples

Standard library documentation: anydbm