shelve module and ZODB package of the prior sections are
powerful tools. Both allow scripts to throw nearly arbitrary Python
objects on a keyed-access file and load them back later—in a single
step for shelves and with a small amount of administrative code for
ZODB. Especially for applications that record highly structured data,
object databases can be convenient and efficient—there is no need to
split and later join together the parts of large objects, and stored
data is processed with normal Python syntax because it is normal
Shelves and ZODB aren’t relational database systems, though; objects (records) are accessed with a single key, and there is no notion of SQL queries. Shelves, for instance, are essentially databases with a single index and no other query-processing support. Although it’s possible to build a multiple-index interface to store data with multiple shelves, it’s not a trivial task and requires manually coded extensions.
ZODB supports some types of searching beyond shelve (e.g., its cataloging feature), and persistent objects may be traversed with all the power of the Python language. However, neither shelves nor ZODB object-oriented databases provide the full generality of SQL queries. Moreover, especially for data that has a naturally tabular structure, relational databases may sometimes be a better fit.
For programs that can benefit from the power of SQL, Python also broadly supports relational database management systems ...