Mastering Object-Oriented Python - Second Edition

Incremental computation with __setattr__()

We can create classes which use __setattr__() to detect changes in attribute values. This can lead to incremental computation. The idea is to build derived values after initial attribute values have been set.

Note the complexity of having two senses of attribute setting.

• The client's view: An attribute can be set and other derived values may be computed. In this case, a sophisticated __setattr__() is used.
• The internal view: Setting an attribute must not result in any additional computation. If additional computation is done, this leads to an infinite recursion of setting attributes and computing derived values from those attributes. In this case, the fundamental __setattr__() method of the superclass ...