How to Ace the Google Interview: Ultimate Guide(byte-by-byte.com)

  # super.init omitted for brevity
  class Expession: # abstract base class
      def eval(): pass

  class Literal(Expession):
      def __init__(self, val): self.value = val        
      def eval(self): return self.value

  lit1 = Literal(8)
  print(lit1.eval()) # prints 8

  class UnaryExpr(Expession): pass # base class for unary expressions

  class Negate(UnaryExpr):
      def __init__(self, expr): self.expression = expr
      def eval(self): return - self.expression.eval()

  lit2 = Literal(13)
  neg = Negate(lit2)
  print(neg.eval()) # prints -13

  class BinaryExpression: pass # base class for binary expressions

  # Note that subclasses Add and Multiply have the same
  # __init__ code so I should hoist that into the BinaryExpression
  # base class but for clarity I'm leaving it in the subclasses

  class Add(BinaryExpression):
      def __init__(self, leftExpr, rightExpr):
          self.leftExpression = leftExpr
          self.rightExpression = rightExpr
      def eval(self):
          return self.leftExpression.eval() + self.rightExpression.eval()

  add2literals = Add(lit1, lit2) # 8 + 13
  print(add2literals.eval()) # prints 21

  class Multiply(BinaryExpression):
      def __init__(self, leftExpr, rightExpr):
          self.leftExpression = leftExpr
          self.rightExpression = rightExpr
      def eval(self): return self.leftExpression.eval() * self.rightExpression.eval()

  mult2literals = Multiply(lit1, lit2) # 8 * 13
  print(mult2literals.eval()) # prints 104

  # now let's make a complex expression, say (7 + 2) * (-4)
  # Doing this by hand but a parser would build this from that
  # string
  expr = Multiply(
      Add(Literal(7), Literal(2))
      ,
      Negate(Literal(4)))
  print(expr.eval()) # prints -36