EnumEditor unable to deduce allowed values from Enum trait

[kitchoi]

To reproduce:

from traits.api import HasTraits, Enum

from traitsui.api import Item, View, EnumEditor


class EnumEditorDemo(HasTraits):
    """ Defines the main EnumEditor demo class. """

    name_list = Enum('A-495', 'A-498', 'R-1226', 'TS-17', 'TS-18',
                     'Foo', 12345, (11, 7), None)


    def default_traits_view(self):
        return View(
            Item(
                'name_list', style='simple',
                editor=EnumEditor(),
                label='Simple'
            ),
        )

if __name__ == '__main__':
    EnumEditorDemo().configure_traits()

We get this error:

Traceback (most recent call last):
  File "examples/demo/Standard_Editors/dummy.py", line 31, in <module>
    demo.configure_traits()
  File "/Users/kchoi/ETS/traits/traits/has_traits.py", line 2083, in configure_traits
    args,
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/toolkit.py", line 233, in view_application
    context, view, kind, handler, id, scrollable, args
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/view_application.py", line 85, in view_application
    context, view, kind, handler, id, scrollable, args
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/view_application.py", line 127, in __init__
    args=self.args,
  File "/Users/kchoi/ETS/traitsui/traitsui/view.py", line 463, in ui
    ui.ui(parent, kind)
  File "/Users/kchoi/ETS/traitsui/traitsui/ui.py", line 247, in ui
    self.rebuild(self, parent)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/toolkit.py", line 163, in ui_live
    ui_live.ui_live(ui, parent)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/ui_live.py", line 44, in ui_live
    _ui_dialog(ui, parent, BaseDialog.NONMODAL)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/ui_live.py", line 66, in _ui_dialog
    BaseDialog.display_ui(ui, parent, style)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/ui_base.py", line 285, in display_ui
    ui.owner.init(ui, parent, style)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/ui_live.py", line 226, in init
    self.add_contents(panel(ui), bbox)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/ui_panel.py", line 261, in panel
    panel = _GroupPanel(content[0], ui).control
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/ui_panel.py", line 603, in __init__
    layout = self._add_items(content, inner)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/ui_panel.py", line 877, in _add_items
    editor.prepare(inner)
  File "/Users/kchoi/ETS/traitsui/traitsui/editor.py", line 241, in prepare
    self.init(parent)
  File "/Users/kchoi/ETS/traitsui/traitsui/qt4/enum_editor.py", line 168, in init
    control.addItems(self.names)
TypeError: addItems(self, Iterable[str]): argument 1 has unexpected type 'NoneType'

If I remove this line and let traitsui cleverly guess the editor:

                editor=EnumEditor(),    # remove this

Then the UI can be launched with the expected behaviour.

Similarly, if I have the allowed values in a separate trait, this raises the same error

    name_list = Enum(values="allowed_values")
    allowed_values = List(["A-495", "A-498"])

    def default_traits_view(self):
        return View(
            Item(
                'name_list', style='simple',
                editor=EnumEditor(),
                label='Simple'
            ),
        )

I need to do this instead:

                editor=EnumEditor(values="allowed_values"),

Given the name_list trait already specifies where the allowed values come from, it seems unnecessary to have to tell the editor again.

[corranwebster]

I think a lot of this is because the default EnumEditor created by the BaseEnum trait is fairly sophisticated:

https://github.com/enthought/traits/blob/master/traits/trait_types.py#L2016-L2022

You need to replicate a lot of that to get the same behaviour from a custom one; in particular, you can't assume that this EnumEditor will always be used with the same underlying Enum (or even have access to it at factory creation time).

[rkern]

The general rule is that when you specify the Item(name, editor=...), that editor doesn't inspect the trait for anything. It does no magic. editor=EnumEditor() means editor=EnumEditor(), no more. This is typically a good thing. If it did inspect the trait, it would be unclear when the arguments you passed to EnumEditor were meant to override what it could inspect from the trait and when they were expected to be left as what you specified (or deliberately omitted from the constructor).

The way that information gets transferred from the trait declaration to an editor is when no editor=... is specified in the Item. Then the trait is asked to create_editor() with the appropriate configuration.

[kitchoi]

I think a lot of this is because the default EnumEditor created by the BaseEnum trait is fairly sophisticated:
https://github.com/enthought/traits/blob/master/traits/trait_types.py#L2016-L2022

Interesting! For my understanding, may I ask where is this create_editor called? I have having difficulties finding it...

[corranwebster]

It's called in the get_editor method of the BaseTraitHandler (which is the ABC of all TraitTypes): https://github.com/enthought/traits/blob/master/traits/base_trait_handler.py#L146

which is in turn called in the ui_panel.py module which is responsible for building the concrete UI:

traitsui/traitsui/qt4/ui_panel.py

Line 843 in e032bcc

editor_factory = trait.get_editor().trait_set(