eval.py

class Expr:
  def eval(self, state):
    pass

class Var(Expr):
  def __init__(self, name):
    self.name = name

  def eval(self, state):
    return state[name]
  
class BinOp(Expr):
  def __init__(self, op, left, right):
    self.op = op # here: a binary lambda function, but can be anything
    self.left = left
    self.right = right
  
  def eval(self, state):
    return op(left.eval(state), right.eval(state))