Traits internals

This section of the documentation is intended both for those developing Traits itself, and for those using Traits who would like a better understanding of the Traits internals.


The runtime behavior of Traits is governed by two key classes: CTrait and HasTraits. Each of these classes inherits from a superclass implemented in the ctraits C extension module: CTrait inherits from cTrait, while HasTraits inherits from CHasTraits.

The CHasTraits and cTrait base classes are not intended for direct use and do not provide a complete, coherent API: that API is provided by their respective subclasses HasTraits and CTrait. The existence and precise semantics of the base classes should be regarded as implementation details. In what follows, we’ll frequently refer to behavior of HasTraits and CTrait even when the behavior we’re referring to is actually implemented by CHasTraits or cTrait.

We’ll examine both of these classes, along with some of their hidden state, in more detail below.

Key class: HasTraits

The primary purpose of the HasTraits class is to override Python’s normal rules for attribute access. It does this by overriding the __getattribute__, __setattr__ and __delattr__ special methods. At C level, this is done through providing tp_getattro and tp_setattro “slots” to the CHasTraits PyTypeObject: tp_getattro controls attribute retrieval operations, while tp_setattro is called for both attribute set and attribute deletion operations.

To support trait get and set operations, a HasTraits object (that is, in normal use, an instance of a user-defined subclass of HasTraits) has two key pieces of state: a dictionary of class traits, and a second dictionary of instance traits. Each dictionary is a mapping from trait attribute names to CTrait objects. A CTrait object encapsulates the rules for getting and setting the corresponding attribute, as well as providing an attachment point for trait notifications.

In addition to these two dictionaries, a HasTraits object has a __dict__ that stores attribute values for that object in the normal way.

Analogously to class variables and instance variables in a normal Python class, the class traits dictionary for a given HasTraits subclass is shared between instances of that class, while the instance traits dictionary is specific to a particular HasTraits instance. (As an internal optimization, the instance traits dictionary is created on demand when first needed, while the class traits dictionary is always present for each instance.)

For introspection and debugging purposes, both of these dictionaries can be retrieved directly, using the _instance_traits() and _class_traits() methods. However, it’s not recommended to use either of these methods in production code, and you should be especially careful when modifying either of these dictionaries.

For example, consider the following HasTraits subclass:

from traits.api import Float, HasTraits, Str

class Ingredient(HasTraits):
    """ An ingredient in a recipe listing. """

    #: The name of the ingredient.
    name = Str()

    #: Ingredient quantity.
    quantity = Float()

Here’s what happens when we create an instance of this ingredient and inspect the instance and class trait dictionaries:

>>> eggs = Ingredient(name="eggs", quantity=12)
>>> eggs._instance_traits()
>>> eggs._class_traits()
    'name': <traits.ctrait.CTrait object at 0x1020cd400>,
    'quantity': <traits.ctrait.CTrait object at 0x1020cd360>,
    'trait_added': <traits.ctrait.CTrait object at 0x1020cd4a0>,
    'trait_modified': <traits.ctrait.CTrait object at 0x1020079a0>,
>>> eggs.__dict__
{'name': 'eggs', 'quantity': 12.0}

Note that the actual values for the ingredient are stored in the __dict__ as usual, not in the CTrait objects.

If we create a second ingredient, it shares class traits (but not instance traits) with the first one:

>>> flour = Ingredient(name="flour", quantity=3.5)
>>> flour._class_traits() is eggs._class_traits()
>>> flour._instance_traits() is eggs._instance_traits()

Key class: CTrait

A CTrait object has two main purposes:

  • It encapsulates the rules for getting and setting a traited attribute on a HasTraits object.

  • It provides an attachment point for trait notifiers.

The hidden state for a CTrait instance is encapsulated in the trait_object C struct in the ctraits source. There are several interesting fields, not all of which are exposed at Python level.

Of particular interest are the getattr and setattr fields, which hold pointers to C functions that act as the entry points for attribute access via the given trait. See below for a fuller description of attribute access mechanics.

Attribute retrieval

When evaluating for a HasTraits object obj, the following sequence of steps occurs (see has_traits_getattro in the C source):

  • The name is looked up in obj.__dict__. If found, the corresponding value is returned immediately.

  • If name is not found in obj.__dict__, we look first for an instance trait named name, and then for a class trait named name. Thus an instance trait with a given name will shadow a class trait with the same name.

  • If a matching trait is found, its getattr field is invoked to retrieve the trait’s value for the given object.

  • If no matching trait is found, we try to access the attribute value using Python’s usual attribute rules (via the PyObject_GenericGetAttr C-API call).

  • Finally, if the PyObject_GenericGetAttr call fails, we invoke the prefix trait machinery to get a new CTrait object, and use that new trait to get a value.

Note that the above sequence of steps applies to method access as well as attribute access. Note also that there’s no mechanism to automatically search for CTrait objects in superclasses of the immediate HasTraits subclass.

Attribute set operations

The rules for setting an attribute (evaluating = value for a HasTraits object obj) are analogous to those for attribute retrieval. The starting point is has_traits_setattro in the source.

  • First we look for the name name in obj._instance_traits(), and retrieve the corresponding CTrait instance if present.

  • If no matching entry is found, we then look up name in obj._class_traits(), and again retrieve the corresponding CTrait.

  • If still not found, we invoke the prefix trait machinery to get a new CTrait object. By default, this goes through the HasTraits.__prefix_trait__ method (which is implemented in Python), and this may still fail with an exception.

  • If one of the above steps succeeded, we now have a CTrait object, and its setattr function is invoked (passing along the trait object, obj, name and value) to perform the actual attribute set operation.

Attribute deletion

Attribute deletion (del goes through the same code path as attribute set operations. Most CTrait types do not permit deletion.

Traits Containers

Traits has the ability to watch for changes in standard Python containers: lists, dictionaries and sets. To achieve this Traits provides special subclasses of the standard Python classes that can validate elements and can fire trait notifications when the contents change. These classes are, respectively, TraitList, TraitDict and TraitSet (not to be confused with the deprecated Trait Handlers of the same names).

In addition to being able to take an appropriate value to initialize the container (such as a sequence or mapping), these container subclasses also take keyword-only arguments for validators (either a single item validator for TraitList and TraitSet, or key and value validators for TraitDict) and notifiers.

These classes were introduced in Traits 6.1 and the implementation details described here are provisional and may change.

Backwards Compatibility

In practice, most traits containers are instances of TraitListObject, TraitDictObject and TraitSetObject which are subclasses that are created by the validators of List, Dict and Set traits respectively and supplied with appropriate element validators that wrap the standard trait validators so that constructs like List(Str) can work.

These objects are designed to be API compatible with the classes of the same name from Traits 6.0 and before, and so have different constructors and may behave slightly differently from the base classes in some cases.

In particular TraitListObject classes can have restrictions on their length, which is not part of the base TraitList API.

All of these backward compatibility classes are strongly tied to a particular object and trait, and are not designed to operate as stand-alone entities. These relationships are required to support the object.*_items-style event traits, but complicate the implementation, for example by having to use weak references to the object, and having to take this additional structure into account when serializing and deserializing.

It is a long-term goal to phase out the use of these backwards compatibility classes.

Container Validators

Container validators are callables that are expected to take a single value and either return a validated value or raise a TraitError. The default validators simply pass the input value directly through, but validators can potentially do much more. Validators are expected to be idempotent: validate(validate(x)) == validate(x) should hold as long as validate(x) does not raise an error.

For example, the following validator casts the list items to integers, raising a TraitError if that fails:

def int_validator(value):
        return int(value)
    except ValueError:
        raise TraitError(
            "List items must be castable to an int, but a value %r was specified."
            % value

So if we were to use this as the validator for a TraitList we would get the following behaviour:

>>> int_list = TraitList(item_validator=int_validator)
>>> int_list.append("5")
>>> int_list
>>> int_list.extend([3, 5, "aaaarrrghh"])
TraitError: List items must be castable to an int, but a value 'aaaarrrghh' was specified.

In Traits 6.1 validation is not done uniformly before performing operations to keep behaviour the same as Traits 6.0 and earlier, so results of operations can sometimes be surprising:

>>> int_list.append("6")
>>> int_list.remove("6")
ValueError: list.remove(x): x not in list

This is expected to be resolved in a future traits version by providing clear guidance to users about when validation is performed, and possibly changing the behaviour.

Container Notifiers

Container objects also have a list of notifiers that fire when the contents of the container change. Like trait notifiers, container notifiers are low-level callbacks that are used by the higher-level, more user-friendly observer and listener systems. They have a fixed signature, which is slightly different for lists, dicts and sets, but in all cases starts with the container object itself. Notifiers are called in the order that they appear in the notifiers list and should not mutate the parameters that they have been passed.

List notifiers must take 4 arguments: the trait_list object, the index value or slice that identifies where the change occurred, a list of removed elements, and a list of added elements. The TraitList methods make an attempt to normalize indices and slices to make things easier for notification writers.

Dict notifiers expect 4 arguments: the trait_dict object, a dictionary of removed items, a dictionary of added elements, and a dictionary of changed items, where the values are the old values held in the keys.

Set notifiers expect 3 arguments: the trait_set object, the set of removed elements, and the set of added elements.

Users should not usually need to interact with the container notifiers directly, just as they do not usually need to interact with trait notifiers.