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.. _tips-tricks-and-gotchas:
========================
Tips, Tricks and Gotchas
========================
Getting and Setting Model View Elements
---------------------------------------
For some applications, it can be necessary to retrieve or manipulate the View
objects associated with a given model object. The HasTraits class defines two
methods for this purpose: trait_views() and trait_view().
.. _trait-views:
trait_views()
`````````````
The trait_views() method, when called without arguments, returns a list
containing the names of all Views defined in the object's class. For example, if
**sam** is an object of type SimpleEmployee3 (from
:ref:`Example 6 <example-6-defining-multiple-view-objects-in-a-hastraits-class>`),
the method call ``sam.trait_views()`` returns the list ``['all_view',
'traits_view']``.
Alternatively, a call to :samp:`trait_views({view_element_type})` returns a list
of all named instances of class *view_element_type* defined in the object's
class. The possible values of *view_element_type* are:
- :term:`View`
- :term:`Group`
- :term:`Item`
- :term:`ViewElement`
- ViewSubElement
Thus calling ``trait_views(View)`` is identical to calling ``trait_views()``.
Note that the call ``sam.trait_views(Group)`` returns an empty list, even though
both of the Views defined in SimpleEmployee contain Groups. This is because only
*named* elements are returned by the method.
Group and Item are both subclasses of ViewSubElement, while ViewSubElement and
View are both subclasses of ViewElement. Thus, a call to
``trait_views(ViewSubElement)`` returns a list of named Items and Groups, while
``trait_views(ViewElement)`` returns a list of named Items, Groups and Views.
trait_view()
````````````
The trait_view() method is used for three distinct purposes:
- To retrieve the default View associated with an object
- To retrieve a particular named ViewElement (i.e., Item, Group or View)
- To define a new named ViewElement
For example:
- ``obj.trait_view()`` returns the default View associated with object *obj*.
For example, ``sam.trait_view()`` returns the View object called
``traits_view``. Note that unlike trait_views(), trait_view() returns the
View itself, not its name.
- ``obj.trait_view('my_view')`` returns the view element named ``my_view``
(or None if ``my_view`` is not defined).
- ``obj.trait_view('my_group', Group('a', 'b'))`` defines a Group with the name
``my_group``. Note that although this Group can be retrieved using
``trait_view()``, its name does not appear in the list returned by
``traits_view(Group)``. This is because ``my_group`` is associated with
*obj* itself, rather than with its class.
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