python - 尝试 assertAlmostEqual/assertEqual 时不支持的操作数类型

Question

我正在尝试测试两个对象是否相等。这个对象的类型是Point，是ROS(Robot Operating System)定义的一个类。我有以下测试:

def test_when_getting_position_after_2s_then_position_at_2s_is_returned(self):
    self.expected_position.x = -self.radius
    self.expected_position.y = 0
    self.assertAlmostEqual(
        self.expected_position,
        self.trajectory.get_position_at(2))

我正在使用 unittest，当我尝试断言它们是否几乎相等时，我收到一条错误消息:

TypeError: unsupported operand type(s) for -: 'Point' and 'Point'

我在使用 assertEqual 时遇到同样的错误，我知道我可以这样做:

self.assertAlmostEqual(self.expected_position.x, self.trajectory.get_position_at(1).x)
self.assertAlmostEqual(self.expected_position.y, self.trajectory.get_position_at(1).y)

但是，我希望能够断言位置而不是特定字段。我怎样才能做到这一点？

编辑:异常的完整回溯是:

Error
Traceback (most recent call last):
  File "/usr/lib/python2.7/unittest/case.py", line 329, in run
    testMethod()
  File "/home/m/turtlebot_ws/src/trajectory_tracking/src/test/trajectory/test_astroid_trajectory.py", line 26, in test_when_getting_position_after_1s_then_position_at_1s_is_returned
    self.assertAlmostEqual(self.expected_position, self.trajectory.get_position_at(1))
  File "/usr/lib/python2.7/unittest/case.py", line 554, in assertAlmostEqual
    if round(abs(second-first), places) == 0:
TypeError: unsupported operand type(s) for -: 'Point' and 'Point'

Answer 1

assertAlmostEqual(a, b) 要求 abs(a - b) 有效，但您没有为 Point 类型定义减法运算符，因此出现错误.

class Point(object):
    ...
    def __sub__(self, other):   # <-- define the subtraction operator so `a - b` is valid
        return Vector(self.x - other.x, self.y - other.y)

class Vector(object):
    ...
    def __abs__(self):    # <-- define the absolute function so `abs(v)` is valid
        return (self.x*self.x + self.y*self.y)**0.5

---

如果您不能在类定义中提供 __sub__，您可以使用 monkey-patching 在您的测试用例中提供它。

def sub_point(self, other):
    return complex(self.x - other.x, self.y - other.y)
    # ^ for simplicity we abuse a complex number as a 2D vector.

Point.__sub__ = sub_point