This is different from similar questions I have found on this site as the code in question doesn't have any operators or brackets before the character limit to split easily on. I have several long lines in python without operators or brackets before 79 characters. As an example:
With your particular example, I expect if you first solved the problem of "make this code readable", fitting in 79 characters would come naturally.
That said, you can add brackets:
(self.caller.parent.parent .parent.caller.parent.bar .ids.actionview.remove_widget( self.caller.parent.parent .parent.caller.parent .bar.ids.actionview.startbutton) )