python - 尝试在 Python 中使用运算符方法时出现类型错误

Question

我在更广泛的 python 脚本中使用运算符方法时遇到问题。为了缩小问题范围，我创建了以下示例代码。

class Car:
   def __init__(self, Name, Speed):
      self.Name = Name
      self.Speed = Speed
      self.PrintText()

   def PrintText(self):
      print("My %s runs %s km/h." % (self.Name, self.Speed))

   def GetName(self):
      return self.Name

   def GetSpeed(self):
      return self.Speed

   def __sub__(self, other):
      try: # Try with assumption that we have to deal with an instance of AnotherClass
         a = self.GetSpeed()
      except: # If it doesn't work let's assume we have to deal with a float
         a = self

      try: # Try with assumption that we have to deal with an instance of AnotherClass
         b = other.GetSpeed()
      except: # If it doesn't work let's assume we have to deal with a float
         b = other

      return a - b

Car1 = Car("VW", 200.0)
Car2 = Car("Trabant", 120.0)
print("")

Difference = Car1 - Car2
print("The speed difference is %g km/h." % Difference)
print("")

Difference = Car2 - 6.0
print("My %s is %g km/h faster than a pedestrian." % (Car2.GetName(), Difference))
print("")

Difference = 250.0 - Car1
print("I wish I had a car that is %g km/h faster than the %s." % (Difference, Car1.GetName()))

输出是:

My VW runs 200.0 km/h.
My Trabant runs 120.0 km/h.

The speed difference is 80 km/h.

My Trabant is 114 km/h faster than a pedestrian.

Traceback (most recent call last):
  File "test.py", line 41, in <module>
    Difference = 250.0 - Car1
TypeError: unsupported operand type(s) for -: 'float' and 'instance'

如何解决当第一个对象是 float 时出现的问题？

Answer 1

您需要使用反转操作数定义减法(代码本身已修复//您的问题，但可能需要更多清理):

   def __rsub__(self, other):
        a = self.GetSpeed()

        try: # Try with assumption that we have to deal with an instance of AnotherClass
         b = other.GetSpeed()
        except AttributeError: # If it doesn't work let's assume we have to deal with a float
         b = other

        return b - a

现在你的输出:

My VW runs 200.0 km/h.
My Trabant runs 120.0 km/h.

The speed difference is 80 km/h.

My Trabant is 114 km/h faster than a pedestrian.

I wish I had a car that is 50 km/h faster than the VW.