Traits User Manual
Previous: 3.1.2 The name Parameter Table of Contents Next: 3.1.4 Dynamic Handler Special Cases
Traits User Manual
1 Introduction
1.1 What Are Traits?
1.2 Background
2 Defining Traits: Initialization and Validation
2.1 Predefined Traits
2.1.1 Predefined Traits for Simple Types
2.1.2 Other Predefined Traits
2.2 Trait Metadata
2.2.1 Internal Metadata Attributes
2.2.2 Recognized Metadata Attributes
2.2.3 Accessing Metadata Attributes
3 Trait Notification
3.1 Dynamic Notification
3.1.1 Example of a Dynamic Notification Handler
3.1.2 The name Parameter
3.1.3 Notification Handler Signatures
3.1.4 Dynamic Handler Special Cases
3.2 Static Notification
3.2.1 Handler Decorator
3.2.2 Specially-named Notification Handlers
3.2.3 Attribute-specific Handler Signatures
3.2.4 General Static Handler Signatures
3.3 Trait Events
3.4 Undefined Object
4 Deferring Trait Definitions
4.1 DelegatesTo
4.2 PrototypedFrom
4.3 Keyword Parameters
4.3.1 Prefix Keyword
4.3.2 Listenable Keyword
4.4 Notification with Deferring
5 Custom Traits
5.1 Trait Subclassing
5.1.1 Defining a Trait Type
5.1.2 Defining a Trait Property
5.1.3 Other TraitType Members
5.2 The Trait() Factory Function
5.2.1 Trait () Parameters
5.2.2 Mapped Traits
5.3 Trait Handlers
5.3.1 TraitPrefixList
5.3.2 TraitPrefixMap
5.4 Custom Trait Handlers
5.4.1 Example Custom Trait Handler
6 Advanced Topics
6.1 Initialization and Validation Revisited
6.1.1 Dynamic Initialization
6.1.2 Overriding Default Values in a Subclass
6.1.3 Reusing Trait Definitions
6.1.4 Trait Attribute Definition Strategies
6.1.5 Type-Checked Methods
6.2 Interfaces
6.2.1 Defining an Interface
6.2.2 Implementing an Interface
6.2.3 Using Interfaces
6.3 Adaptation
6.3.1 Defining Adapters
6.3.2 Using Adapters
6.3.3 Controlling Adaptation
6.4 Property Traits
6.4.1 Property Factory Function
6.4.2 Caching a Property Value
6.5 Persistence
6.5.1 Pickling HasTraits Objects
6.5.2 Predefined Transient Traits
6.5.3 Overriding __getstate__()
6.5.4 Unpickling HasTraits Objects
6.5.5 Overriding __setstate__()
6.6 Useful Methods on HasTraits
6.6.1 add_trait()
6.6.2 clone_traits()
6.6.3 set()
6.6.4 add_class_trait()
6.7 Performance Considerations of Traits

3.1.3        Notification Handler Signatures

The handler passed to on_trait_change() or on_trait_event() can have any one of the following signatures:

handler()

handler(new)

handler(name, new)

handler(object, name, new)

handler(object, name, old, new)

These signatures use the following parameters:

·    object—The object whose trait attribute changed.

·    name—The attribute that changed. If one of the objects in a sequence is a List or Dict, and its membership changes, then this is the name of the trait that reference it, with _items appended. For example, if the handler is monitoring 'foo.bar.baz', where bar is a List, and an item is added to bar, then the value of the name parameter is bar_items.

·    new—The new value of the trait attribute that changed. For changes to List and Dict objects, this is a list of items that were added.

·    old—The old value of the trait attribute that changed. For changes to List and Dict object, this is a list of items that were deleted. For event traits, this is Undefined.

If the handler is a bound method, it also implicitly has self as a first argument.

Previous: 3.1.2 The name Parameter Table of Contents Next: 3.1.4 Dynamic Handler Special Cases
Traits User Manual