class Vector 的创建

Question

创建一个向量的class，可以让vector相加，相减， 乘以一个系数， 或者清空成为（0，0，0）

from math import sqrt

class Vector:
    """
    A Vector is a 3-tuple of (x,y,z) coordinates.
    """
    def __init__(self,x,y,z):
        self._x = x
        self._y = y
        self._z = z
    def __repr__(self):
        return '{%.3g} ,{%.3g} ,{%.3g}'%(self._x, self._y, self._z)
    def __str__(self):
        return '({},{},{})'.format(self._x, self._y, self._z)
    def __add__(self,other):
        return Vector(self._x + other._x, self._y + other._y, self._z + other._z)
    def __sub__(self,other):
        return Vector(self._x - other._x, self._y - other._y, self._z - other._z)
    def norm(self):
        result = sqrt(self._x**2 + self._y**2 + self._z**2)
        return result
    def __mul__(self,other):
        return Vector(self._x * other, self * other, self._z * other)
    def x(self):
        return self._x
    def y(self):
        return self._y
    def z(self):
        return self._z
    def clear(self):
        return Vector(0 ,0 ,0)

以下是我test的结果

v1 = Vector(1, 1, 1)
v2 = Vector(2, 2, 2)
v3 = Vector(3, 3, 3)

assert str(v1) == '(1,1,1)'
assert str(v2) == '(2,2,2)'

assert Vector(1, 2, 3) == Vector(1, 2, 3)
assert Vector(1, 2, 3) != Vector(1.001, 2.001, 3.001)
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-4-158bfce1869b> in <module>()
----> 1 assert Vector(1, 2, 3) == Vector(1, 2, 3)
      2 assert Vector(1, 2, 3) != Vector(1.001, 2.001, 3.001)

AssertionError: 


assert v1 + v2 == v3
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-8-aee75f985c01> in <module>()
----> 1 assert v1 + v2 == v3
      

AssertionError: 


assert v3 - v2 == v1
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-6-b6adfd2202df> in <module>()
----> 1 assert v3 - v2 == v1

AssertionError: 


assert v3 == v1 * 3
#Error 信息太多 不复制出来了


assert round(v2.norm(), 10) == round(sqrt(12), 10)


v1.clear()
assert v1 == Vector(0,0,0)
---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-9-188a44181827> in <module>()
      1 v1.clear()
----> 2 assert v1 == Vector(0,0,0)

AssertionError: 

问题到底出在哪里？
比如向量相加，我print v1+v2 是（3，3，3）v3 也是（3，3，3），为什么还是assertion error？

Answer 1

你需要定义 __eq__：

    def __eq__(self, a):
        return self._x == a._x and self._y == a._y and self._z == a._z

原因在此：

The behavior of the default equality comparison, that instances with different identities are always unequal, may be in contrast to what types will need that have a sensible definition of object value and value-based equality.
via https://docs.python.org/3/ref...

也就是说，没定义 __eq__ 时，v1 == v2 相当于 id(v1) == id(v2)，当然是 False 了。