Just to add a detail: using ComputedAttribute on a RichText field results in a view error. Using the property decorator as @jaroel showed, worked for me in this situation. Edit: fails when adding a new piece of content.
I tried to debug the converter in plone.app.textfield.widget but was not successful.
Traceback (innermost last):
Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
Module ZPublisher.WSGIPublisher, line 371, in publish_module
Module ZPublisher.WSGIPublisher, line 266, in publish
Module ZPublisher.mapply, line 85, in mapply
Module ZPublisher.WSGIPublisher, line 63, in call_object
Module plone.z3cform.layout, line 63, in __call__
Module plone.z3cform.layout, line 47, in update
Module plone.dexterity.browser.edit, line 55, in update
Module plone.z3cform.fieldsets.extensible, line 65, in update
Module plone.z3cform.patch, line 30, in GroupForm_update
Module z3c.form.group, line 141, in update
Module z3c.form.group, line 52, in update
Module z3c.form.group, line 48, in updateWidgets
Module z3c.form.field, line 277, in update
Module plone.app.textfield.widget, line 42, in update
Module z3c.form.browser.textarea, line 37, in update
Module z3c.form.browser.widget, line 171, in update
Module Products.CMFPlone.patches.z3c_form, line 47, in _wrapped
Module z3c.form.widget, line 132, in update
Module plone.app.textfield.widget, line 99, in toWidgetValue
ValueError: Can not convert <ComputedAttribute object at 0x7f4390e3c660> to an IRichTextValue
With the @property decorator adding new content results in:
Traceback (innermost last):
Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
Module ZPublisher.WSGIPublisher, line 371, in publish_module
Module ZPublisher.WSGIPublisher, line 266, in publish
Module ZPublisher.mapply, line 85, in mapply
Module ZPublisher.WSGIPublisher, line 63, in call_object
Module plone.z3cform.layout, line 63, in __call__
Module plone.z3cform.layout, line 47, in update
Module plone.dexterity.browser.add, line 138, in update
Module plone.z3cform.fieldsets.extensible, line 65, in update
Module plone.z3cform.patch, line 30, in GroupForm_update
Module z3c.form.group, line 145, in update
Module plone.app.z3cform.csrf, line 22, in execute
Module z3c.form.action, line 98, in execute
Module z3c.form.button, line 315, in __call__
Module z3c.form.button, line 170, in __call__
Module plone.dexterity.browser.add, line 116, in handleAdd
Module z3c.form.form, line 263, in createAndAdd
Module plone.dexterity.browser.add, line 84, in create
Module plone.dexterity.browser.add, line 198, in _applyChanges
Module z3c.form.datamanager, line 91, in set
AttributeError: can't set attribute