The Traits project is at the center of all Enthought Tool Suite
development and has changed the mental model used at Enthought for
programming in the already extremely efficient Python programming
language.

The Traits project allows Python programmers to use a special kind
of type definition called a trait, which gives object attributes
some additional characteristics, such as Initialization, Validation,
Delegation, Notification and Visualization.

A class can freely mix trait-based attributes with normal Python
attributes, or can opt to allow the use of only a fixed or open set
of trait attributes within the class. Trait attributes defined by
a classs are automatically inherited by any subclass derived from
the class.

WWW: https://pypi.python.org/pypi/Traits/