code: def set_current_filename(self, filename, focus=True):
          
          index = self.has_filename(filename)
          if index is not None:
              if focus:
                  self.set_stack_index(index)
              editor = self.data[index].editor
              if focus:
                  editor.setFocus()
              else:
                  self.stack_history.remove_and_append(index)
  
              return editor
docs: Set current filename and return the associated editor instance.