Programming Python, Second Edition

DBM Files

Flat files are handy for simple persistence tasks, but are generally geared towards a sequential processing mode. Although it is possible to jump around to arbitrary locations with seek calls, flat files don’t provide much structure to data beyond the notion of bytes and text lines.

DBM files, a standard tool in the Python library for database management, improve on that by providing key-based access to stored text strings. They implement a random-access, single-key view on stored data. For instance, information related to objects can be stored in a DBM file using a unique key per object and later can be fetched back directly with the same key. DBM files are implemented by a variety of underlying modules (including one coded in Python), but if you have Python, you have a DBM.

Using DBM Files

Although DBM filesystems have to do a bit of work to map chunks of stored data to keys for fast retrieval (technically, they generally use a technique called hashing to store data in files), your scripts don’t need to care about the action going on behind the scenes. In fact, DBM is one of the easiest ways to save information in Python -- DBM files behave so much like in-memory dictionaries that you may forget you’re actually dealing with a file. For instance, given a DBM file object:

• Indexing by key fetches data from the file.

• Assigning to an index stores data in the file.

DBM file objects also support common dictionary methods such as keys-list fetches and tests, and key deletions. ...