Programming Python, 3rd Edition

PyForm: A Persistent Object Viewer

Instead of going into additional database interface details that are freely available at Python.org, I’m going to close out this chapter by showing you one way to combine the GUI technology we met earlier in the text with the persistence techniques introduced in this chapter. This section presents PyForm, a Tkinter GUI designed to let you browse and edit tables of records:

• Tables browsed are shelves, DBM files, in-memory dictionaries, or any other object that looks and feels like a dictionary.

• Records within tables browsed can be class instances, simple dictionaries, strings, or any other object that can be translated to and from a dictionary.

Although this example is about GUIs and persistence, it also illustrates Python design techniques. To keep its implementation both simple and type-independent, the PyForm GUI is coded to expect tables to look like dictionaries of dictionaries. To support a variety of table and record types, PyForm relies on separate wrapper classes to translate tables and records to the expected protocol:

• At the top table level, the translation is easy—shelves, DBM files, and in-memory dictionaries all have the same key-based interface.

• At the nested record level, the GUI is coded to assume that stored items have a dictionary-like interface too, but classes intercept dictionary operations to make records compatible with the PyForm protocol. Records stored as strings are converted to and from the dictionary objects on fetches and ...