Custom Traits

The predefined traits such as those described in Predefined Traits are handy shortcuts for commonly used types. However, the Traits package also provides facilities for defining complex or customized traits:

  • Subclassing of traits
  • The Trait() factory function
  • Predefined or custom trait handlers

Trait Subclassing

Starting with Traits version 3.0, most predefined traits are defined as subclasses of traits.trait_handlers.TraitType. As a result, you can subclass one of these traits, or TraitType, to derive new traits. Refer to the Traits API Reference to see whether a particular predefined trait derives from TraitType.

Here’s an example of subclassing a predefined trait class:

# -- Example of subclassing a trait class
from traits.api import BaseInt

class OddInt ( BaseInt ):

    # Define the default value
    default_value = 1

    # Describe the trait type
    info_text = 'an odd integer'

    def validate ( self, object, name, value ):
        value = super(OddInt, self).validate(object, name, value)
        if (value % 2) == 1:
            return value

        self.error( object, name, value )

The OddInt class defines a trait that must be an odd integer. It derives from BaseInt, rather than Int, as you might initially expect. BaseInt and Int are exactly the same, except that Int has a fast_validate attribute, which causes it to quickly check types at the C level, not go through the expense of executing the general validate() method. [6]

As a subclass of BaseInt, OddInt can reuse and change any part of the BaseInt class behavior that it needs to. In this case, it reuses the BaseInt class’s validate() method, via the call to super() in the OddInt validate() method. Further, OddInt is related to BaseInt, which can be useful as documentation, and in programming.

You can use the subclassing strategy to define either a trait type or a trait property, depending on the specific methods and class constants that you define. A trait type uses a validate() method, while a trait property uses get() and set() methods.

Defining a Trait Type

The members that are specific to a trait type subclass are:

  • validate() method
  • post_setattr() method
  • default_value attribute or get_default_value() method

Of these, only the validate() method must be overridden in trait type subclasses.

A trait type uses a validate() method to determine the validity of values assigned to the trait. Optionally, it can define a post_setattr() method, which performs additional processing after a value has been validated and assigned.

The signatures of these methods are:

validate(object, name, value)
post_setattr(object, name, value)

The parameters of these methods are:

  • object: The object whose trait attribute whose value is being assigned.
  • name: The name of the trait attribute whose value is being assigned.
  • value: The value being assigned.

The validate() method returns either the original value or any suitably coerced or adapted value that is legal for the trait. If the value is not legal, and cannot be coerced or adapted to be legal, the method must either raise a TraitError, or calls the error() method to raise a TraitError on its behalf.

The subclass can define a default value either as a constant or as a computed value. To use a constant, set the class-level default_value attribute. To compute the default value, override the TraitType class’s get_default_value() method.

Defining a Trait Property

A trait property uses get() and set() methods to interact with the value of the trait. If a TraitType subclass contains a get() method or a set() method, any definition it might have for validate() is ignored.

The signatures of these methods are:

get(object, name)
set(object, name, value)

In these signatures, the parameters are:

  • object: The object that the property applies to.
  • name: The name of the trait property attribute on the object.
  • value: The value being assigned to the property.

If only a get() method is defined, the property behaves as read-only. If only a set() method is defined, the property behaves as write-only.

The get() method returns the value of the name property for the specified object. The set() method does not return a value, but will raise a TraitError if the specified value is not valid, and cannot be coerced or adapted to a valid value.

Other TraitType Members

The following members can be specified for either a trait type or a trait property:

  • info_text attribute or info() method
  • init() method
  • create_editor() method

A trait must have an information string that describes the values accepted by the trait type (for example ‘an odd integer’). Similarly to the default value, the subclass’s information string can be either a constant string or a computed string. To use a constant, set the class-level info_text attribute. To compute the info string, override the TraitType class’s info() method, which takes no parameters.

If there is type-specific initialization that must be performed when the trait type is created, you can override the init() method. This method is automatically called from the __init__() method of the TraitType class.

If you want to specify a default TraitsUI editor for the new trait type, you can override the create_editor() method. This method has no parameters, and returns the default trait editor to use for any instances of the type.

For complete details on the members that can be overridden, refer to the Traits API Reference sections on the TraitType and BaseTraitHandler classes.

The Trait() Factory Function

The Trait() function is a generic factory for trait definitions. It has many forms, many of which are redundant with the predefined shortcut traits. For example, the simplest form Trait(default_value), is equivalent to the functions for simple types described in Predefined Traits for Simple Types. For the full variety of forms of the Trait() function, refer to the Traits API Reference.

The most general form of the Trait() function is:

traits.traits.Trait(default_value, {type | constant_value | dictionary | class | function | trait_handler | trait }+)[source]

The notation { | | }+ means a list of one or more of any of the items listed between the braces. Thus, this form of the function consists of a default value, followed by one or more of several possible items. A trait defined with multiple items is called a compound trait. When more than one item is specified, a trait value is considered valid if it meets the criteria of at least one of the items in the list.

The following is an example of a compound trait with multiple criteria:

# -- Example of multiple criteria in a trait definition
from traits.api import HasTraits, Trait, Range

class Die ( HasTraits ):

    # Define a compound trait definition:
    value = Trait( 1, Range( 1, 6 ),
                  'one', 'two', 'three', 'four', 'five', 'six' )

The Die class has a value trait, which has a default value of 1, and can have any of the following values:

  • An integer in the range of 1 to 6
  • One of the following strings: ‘one’, ‘two’, ‘three’, ‘four’, ‘five’, ‘six’

Trait () Parameters

The items listed as possible arguments to the Trait() function merit some further explanation.

  • type: See Type.
  • constant_value: See Constant Value.
  • dictionary: See Mapped Traits.
  • class: Specifies that the trait value must be an instance of the specified class or one of its subclasses.
  • function: A “validator” function that determines whether a value being assigned to the attribute is a legal value. Traits version 3.0 provides a more flexible approach, which is to subclass an existing trait (or TraitType) and override the validate() method.
  • trait_handler: See Trait Handlers.
  • trait: Another trait object can be passed as a parameter; any value that is valid for the specified trait is also valid for the trait referencing it.


A type parameter to the Trait() function can be any of the following standard Python types:

  • str or StringType
  • unicode or UnicodeType
  • int or IntType
  • long or LongType
  • float or FloatType
  • complex or ComplexType
  • bool or BooleanType
  • list or ListType
  • tuple or TupleType
  • dict or DictType
  • FunctionType
  • MethodType
  • ClassType
  • InstanceType
  • TypeType
  • NoneType

Specifying one of these types means that the trait value must be of the corresponding Python type.

Constant Value

A constant_value parameter to the Trait() function can be any constant belonging to one of the following standard Python types:

  • NoneType
  • int
  • long
  • float
  • complex
  • bool
  • str
  • unicode

Specifying a constant means that the trait can have the constant as a valid value. Passing a list of constants to the Trait() function is equivalent to using the Enum predefined trait.

Mapped Traits

If the Trait() function is called with parameters that include one or more dictionaries, then the resulting trait is called a “mapped” trait. In practice, this means that the resulting object actually contains two attributes:

  • An attribute whose value is a key in the dictionary used to define the trait.
  • An attribute containing its corresponding value (i.e., the mapped or “shadow” value). The name of the shadow attribute is simply the base attribute name with an underscore appended.

Mapped traits can be used to allow a variety of user-friendly input values to be mapped to a set of internal, program-friendly values.

The following examples illustrates mapped traits that map color names to tuples representing red, green, blue, and transparency values:

# --- Example of a mapped trait
from traits.api import HasTraits, Trait

standard_color = Trait ('black',
              {'black':       (0.0, 0.0, 0.0, 1.0),
               'blue':        (0.0, 0.0, 1.0, 1.0),
               'cyan':        (0.0, 1.0, 1.0, 1.0),
               'green':       (0.0, 1.0, 0.0, 1.0),
               'magenta':     (1.0, 0.0, 1.0, 1.0),
               'orange':      (0.8, 0.196, 0.196, 1.0),
               'purple':      (0.69, 0.0, 1.0, 1.0),
               'red':         (1.0, 0.0, 0.0, 1.0),
               'violet':      (0.31, 0.184, 0.31, 1.0),
               'yellow':      (1.0, 1.0, 0.0, 1.0),
               'white':       (1.0, 1.0, 1.0, 1.0),
               'transparent': (1.0, 1.0, 1.0, 0.0) } )

red_color = Trait ('red', standard_color)

class GraphicShape (HasTraits):
    line_color = standard_color
    fill_color = red_color

The GraphicShape class has two attributes: line_color and fill_color. These attributes are defined in terms of the standard_color trait, which uses a dictionary. The standard_color trait is a mapped trait, which means that each GraphicShape instance has two shadow attributes: line_color_ and fill_color_. Any time a new value is assigned to either line_color or fill_color, the corresponding shadow attribute is updated with the value in the dictionary corresponding to the value assigned. For example:

>>> import mapped
>>> my_shape1 = mapped.GraphicShape()
>>> print my_shape1.line_color, my_shape1.fill_color
black red
>>> print my_shape1.line_color_, my_shape1.fill_color_
(0.0, 0.0, 0.0, 1.0) (1.0, 0.0, 0.0, 1.0)
>>> my_shape2 = mapped.GraphicShape()
>>> my_shape2.line_color = 'blue'
>>> my_shape2.fill_color = 'green'
>>> print my_shape2.line_color, my_shape2.fill_color
blue green
>>> print my_shape2.line_color_, my_shape2.fill_color_
(0.0, 0.0, 1.0, 1.0) (0.0, 1.0, 0.0, 1.0)

This example shows how a mapped trait can be used to create a user-friendly attribute (such as line_color) and a corresponding program-friendly shadow attribute (such as line_color_). The shadow attribute is program-friendly because it is usually in a form that can be directly used by program logic.

There are a few other points to keep in mind when creating a mapped trait:

  • If not all values passed to the Trait() function are dictionaries, the non-dictionary values are copied directly to the shadow attribute (i.e., the mapping used is the identity mapping).
  • Assigning directly to a shadow attribute (the attribute with the trailing underscore in the name) is not allowed, and raises a TraitError.

The concept of a mapped trait extends beyond traits defined via a dictionary. Any trait that has a shadow value is a mapped trait. For example, for the Expression trait, the assigned value must be a valid Python expression, and the shadow value is the compiled form of the expression.

Trait Handlers

In some cases, you may want to define a customized trait that is unrelated to any predefined trait behavior, or that is related to a predefined trait that happens to not be derived from TraitType. The option for such cases is to use a trait handler, either a predefined one or a custom one that you write.

A trait handler is an instance of the traits.trait_handlers.TraitHandler class, or of a subclass, whose task is to verify the correctness of values assigned to object traits. When a value is assigned to an object trait that has a trait handler, the trait handler’s validate() method checks the value, and assigns that value or a computed value, or raises a TraitError if the assigned value is not valid. Both TraitHandler and TraitType derive from BaseTraitHandler; TraitHandler has a more limited interface.

The Traits package provides a number of predefined TraitHandler subclasses. A few of the predefined trait handler classes are described in the following sections. These sections also demonstrate how to define a trait using a trait handler and the Trait() factory function. For a complete list and descriptions of predefined TraitHandler subclasses, refer to the Traits API Reference, in the section on the traits.trait_handlers module.


The TraitPrefixList handler accepts not only a specified set of strings as values, but also any unique prefix substring of those values. The value assigned to the trait attribute is the full string that the substring matches.

For example:

>>> from traits.api import HasTraits, Trait
>>> from traits.api import TraitPrefixList
>>> class Alien(HasTraits):
...   heads = Trait('one', TraitPrefixList(['one','two','three']))
>>> alf = Alien()
>>> alf.heads = 'o'
>>> print alf.heads
>>> alf.heads = 'tw'
>>> print alf.heads
>>> alf.heads = 't'  # Error, not a unique prefix
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "c:\svn\ets3\traits_3.0.3\enthought\traits\", line 1802,
 in validate self.error( object, name, value )
  File "c:\svn\ets3\traits_3.0.3\enthought\traits\", line 175,
in error value )
traits.trait_errors.TraitError: The 'heads' trait of an Alien instance
 must be 'one' or 'two' or 'three' (or any unique prefix), but a value of 't'
 <type 'str'> was specified.


The TraitPrefixMap handler combines the TraitPrefixList with mapped traits. Its constructor takes a parameter that is a dictionary whose keys are strings. A string is a valid value if it is a unique prefix for a key in the dictionary. The value assigned is the dictionary value corresponding to the matched key.

The following example uses TraitPrefixMap to define a Boolean trait that accepts any prefix of ‘true’, ‘yes’, ‘false’, or ‘no’, and maps them to 1 or 0.

# --- Example of using the TraitPrefixMap handler
from traits.api import Trait, TraitPrefixMap

boolean_map = Trait('true', TraitPrefixMap( {
                              'true': 1,
                              'yes':  1,
                              'false': 0,
                              'no':   0 } ) )

Custom Trait Handlers

If you need a trait that cannot be defined using a predefined trait handler class, you can create your own subclass of TraitHandler. The constructor (i.e., __init__() method) for your TraitHandler subclass can accept whatever additional information, if any, is needed to completely specify the trait. The constructor does not need to call the TraitHandler base class’s constructor.

The only method that a custom trait handler must implement is validate(). Refer to the Traits API Reference for details about this function.

Example Custom Trait Handler

The following example defines the OddInt trait (also implemented as a trait type in Defining a Trait Type) using a TraitHandler subclass.

# --- Example of a custom TraitHandler
import types
from traits.api import TraitHandler

class TraitOddInteger(TraitHandler):
    def validate(self, object, name, value):
        if ((type(value) is types.IntType) and
            (value > 0) and ((value % 2) == 1)):
            return value
        self.error(object, name, value)

    def info(self):
        return '**a positive odd integer**'

An application could use this new trait handler to define traits such as the following:

# --- Example of using a custom TraitHandler
from traits.api import HasTraits, Trait, TraitRange
from custom_traithandler import TraitOddInteger

class AnOddClass(HasTraits):
    oddball = Trait(1, TraitOddInteger())
    very_odd = Trait(-1, TraitOddInteger(),
                         TraitRange(-10, -1))

The following example demonstrates why the info() method returns a phrase rather than a complete sentence:

>>> from use_custom_th import AnOddClass
>>> odd_stuff = AnOddClass()
>>> odd_stuff.very_odd = 0
Traceback (most recent call last):
  File "", line 25, in ?
    odd_stuff.very_odd = 0
  File "C:\wrk\src\lib\enthought\traits\", line 1119, in validate
    raise TraitError, excp
traits.traits.TraitError: The 'very_odd' trait of an AnOddClass instance
must be **a positive odd integer** or -10 <= an integer <= -1, but a value
of 0 <type 'int'> was specified.

Note the emphasized result returned by the info() method, which is embedded in the exception generated by the invalid assignment.


[6]All of the basic predefined traits (such as Float and Str) have a BaseType version that does not have the fast_validate attribute.