Traits User Manual
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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       Dynamic Notification

Dynamic notification is useful in cases where a notification handler cannot be defined on the class (or a subclass) whose trait attribute changes are to be monitored, or if you want to monitor changes on certain instances of a class, but not all of them. To use dynamic notification, you define a handler method or function, and then invoke the on_trait_change() or on_trait_event() method to register that handler with the object being monitored. Multiple handlers can be defined for the same object, or even for the same trait attribute on the same object.

The handler registration methods have the following signatures:

obj.on_trait_change(handler, name=None, remove=False, dispatch='same')

obj.on_trait_event(handler, name=None, remove=False, dispatch='same')

·    handler—Specifies the function or bound method to be called whenever the trait attributes specified by the name parameter are modified.

·    name—Specifies trait attributes whose changes trigger the handler being called. If this parameter is omitted or is None, the handler is called whenever any trait attribute of the object is modified. The syntax supported by this parameter is discussed in Section 3.1.2.

·    remove—If True (or non-zero), then handler will no longer be called when the specified trait attributes are modified. In other words, it causes the handler to be “unhooked”.

·    dispatch—String indicating the thread on which notifications must be run. In most cases, it can be omitted. See the Traits API Reference for details on non-default values.

Previous: 3 Trait Notification Table of Contents Next: 3.1.1 Example of a Dynamic Notification Handler
Traits User Manual