code: def all_contents(self):
          
          for collection, _ in self.resolve():
              try:
                  it = iter(collection)
              except TypeError:
                  raise ConfigTypeError(
                      u'{0} must be an iterable, not {1}'.format(
                          self.name, type(collection).__name__
                      )
                  )
              for value in it:
                  yield value
docs: Iterates over all subviews from collections at this view from
          *all* sources. If the object for this view in any source is not
          iterable, then a ConfigTypeError is raised. This method is
          intended to be used when the view indicates a list; this method
          will concatenate the contents of the list from all sources.