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Python Standard Library: Data Storage

10-1

Data Storage

"Unlike mainstream component programming, scripts usually do not

introduce new components but simply "wire" existing ones. Scripts can

be seen as introducing behavior but no new state. /.../ Of course, there

is nothing to stop a "scripting" language from introducing persistent

state — it then simply turns into a normal programming language"

Clemens Szyperski, in Component Software

Overview

Python comes with drivers for a number of very similar database managers, all modeled after Unix's

dbm

library. These databases behaves like ordinary dictionaries, with the exception that you can only

use strings for keys and values (the

shelve

module can handle any kind of value).

Python Standard Library

Python Standard Library: Data Storage

10-2

The anydbm module

This module provides a unified interface to the simple database drivers supported by Python.

The first time it is imported, the

anydbm

module looks for a suitable database driver, testing for

dbhash, gdbm, dbm,

dumbdbm,

in that order. If no such module is found, it raises an

ImportError

exception.

The

open

function is used to open or create a database, using the chosen database handler.

Example: Using the anydbm module

# File:

anydbm-example-1.py

import anydbm

db = anydbm.open("database", "c")

db["1"] = "one"

db["2"] = "two"

db["3"] = "three"

db.close()

db = anydbm.open("database", "r")

for key in db.keys():

print repr(key), repr(db[key])

'2' 'two'

'3' 'three'

'1' 'one'

Python Standard Library: Data Storage

10-3

The whichdb module

This module can be used to figure out which database handler that was used for a given database file.

Example: Using the whichdb module

# File:

whichdb-example-1.py

import whichdb

filename = "database"

result = whichdb.whichdb(filename)

if result:

print "file created by", result

handler = __import__(result)

db = handler.open(filename, "r")

print db.keys()

else:

# cannot identify data base

if result is None:

print "cannot read database file", filename

else:

print "cannot identify database file", filename

db = None

This example used the

__import__

function to import a module with the given name.

Python Standard Library: Data Storage

10-4

The shelve module

This module uses the database handlers to implement persistent dictionaries. A shelve object uses

string keys, but the value can be of any data type, as long as it can be handled by the

pickle

module.

Example: Using the shelve module

# File:

shelve-example-1.py

import shelve

db = shelve.open("database", "c")

db["one"] = 1

db["two"] = 2

db["three"] = 3

db.close()

db = shelve.open("database", "r")

for key in db.keys():

print repr(key), repr(db[key])

'one' 1

'three' 3

'two' 2

The following example shows how to use the

shelve

module with a given database driver.

Example: Using the shelve module with a given database

# File:

shelve-example-3.py

import shelve

import gdbm

def gdbm_shelve(filename, flag="c"):

return shelve.Shelf(gdbm.open(filename, flag))

db = gdbm_shelve("dbfile")

Python Standard Library: Data Storage

10-5

The dbhash module

(Optional). This module provides a

dbm-compatible

interface to the

bsddb

database handler.

Example: Using the dbhash module

# File:

dbhash-example-1.py

import dbhash

db = dbhash.open("dbhash", "c")

db["one"] = "the foot"

db["two"] = "the shoulder"

db["three"] = "the other foot"

db["four"] = "the bridge of the nose"

db["five"] = "the naughty bits"

db["six"] = "just above the elbow"

db["seven"] = "two inches to the right of a very naughty bit indeed"

db["eight"] = "the kneecap"

db.close()

db = dbhash.open("dbhash", "r")

for key in db.keys():

print repr(key), repr(db[key])

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