def on_binop ( self , node ) : # ('left', 'op', 'right') return op2func ( node . op ) ( self . run ( node . left ) , self . run ( node . right ) )
python left to right or right to left evaluation
Binary operator .
def on_binop(self, node):    # ('left', 'op', 'right')
        """Binary operator."""
        return op2func(node.op)(self.run(node.left),
                                self.run(node.right))