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Safari | Python Developer's Handbook -> DBM (Database Managers) Databases





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DBM (Database Managers) Databases

Now, let's look at this other mechanism for storing data. The next modules store data in dbm-style format. This format specifies a simple disk-based storage facility that handles data in a way equivalent to dictionaries. The objects are manipulated by using unique key strings. Each of these modules is an interface to a specific library.

dbm, gdbm, and dbhash are database modules that are part of the standard Python distribution.

Also included with the standard Python distribution is the anydbm module, which is a generic interface to all the dbm-like modules. It uses the modules that are installed.

The dbhash module provides a function that offers a dbm-style interface to access the BSD database library.

All these modules have some behavior in common. For example, to open the files, the following syntax is used by all of them.

					
dbhandle = open(filename [, flag [,mode]])

				

Where, filename is the database filename; flag can have one of the following values: r (read-only access), w (read/write access), c (create the database), n (force the creation of a new database); and mode specifies the file access mode (specific for UNIX systems).

The following operations are supported:

					
dbhandle[key] = value    # Set the value of a given key entry
value = dbhandle[key]    # Get the value of a given key entry
dbhandle.has_key(key)    # Test whether a key exists
dbhandle.keys()          # Returns a list of the current keys available
del dbhandle[key]        # Delete a key
dbhandle.close()         # Close the file

				

For all these dbm-like modules, the keys and the values to be stored must be of type string. Later, you will see a module called shelve with a behavior similar to these dbm-like modules. However, it stores persistent objects.

Each module provides its own exception, which is called modulename. error.

					
>>> import anydbm
>>> try:
        dbhandle = anydbm.open("datafile","r")
    except anydbm.error:
        print "Error while opening file"

Error while opening file
>>>

				

This is a simplified database system based on key/value pairs. Depending on the module and the system, it uses one or two files to store the data (for example, both gdbm and bsddb use a single file).

The disadvantage of this kind of implementation is that it is not portable. The storage format is specific to a particular hardware platform and operating system. Also, it is not designed for large volumes of data. The smaller the file, the better the performance. This is caused by the original specification, which wanted information to be accessed in a single system call. After some interactions, the data file gets very fragmented, full of data holes, which drives the performance to very low indexes. Of course, they are very efficient when you do lots of reads and almost no writes.

If you have a data file but you don't know which database you used to create it, take a look at the whichdb module.

The whichdb module provides a function that guesses which dbm module (dbm, gdbm, or dbhash) should be used to open a specific database. However, using the anydbm module should take care of guessing the format for you.

Another important fact you must know is concerning the storage size limitation of each key/value pair, which is also known as bucket size. The dbm module accepts between 1K and 2K of data. However, both gdbm and bsddb don't have any limitation at all.

dbm Module

The dbm module is a database interface that implements a simple UNIX dbm library access method. dbm objects behave similar to dictionaries in which keys and values must contain string objects. This module allows strings, which can encode any Python object, to be archived in indexed files. dbm is the original implementation of the DBM toolkit. The main function of this module opens a dbm database and returns a dbm object that behaves similar to a dictionary.

						
>>> import dbm
>>> dbhandle = dbm.open("datafile", "c")
>>> dbhandle["animal"] = "parrot"
>>> dbhandle["country"] = "Spain"
>>> dbhandle.close()
>>>
>>> dbhandle = dbm.open("datafile ", "r")
>>> for key in dbhandle.keys():
        print dbhandle[key]
parrot
Spain
>>> db.close()

					

gdbm Module

The gdbm module is similar to the dbm module. However, their files are incompatible. This module provides a GNU/FSF reinterpretation of the GNU dbm library. This module supports multi-user application, it is faster than the dbm module (the performance gets better when the number of records increases), and it was already ported to a larger number of platforms.

Check out the GNU Web site for more details:

http://www.gnu.org/software/gdbm/gdbm.html

						
>>> import gdbm
>>> key = raw_input("key: ")
>>> data = raw_input("value: ")
>>> dbhand
...
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