Question

我想创建两个名称相同的实例，如

a = SomeClass(someAttr1) 
a = SomeClass(someAttr2)

以便新的应该覆盖前一个。

我也试过这个：

a = SomeClass(someAttr1) 
a = None
a = SomeClass(someAttr2)

我尝试了这个，但它没有覆盖以前的实例并将其自身添加，有没有办法做到这一点？

以下是代码：

### Do not change the Location or Campus classes. ###
### Location class is the same as in lecture.     ###
class Location(object):
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def move(self, deltaX, deltaY):
        return Location(self.x + deltaX, self.y + deltaY)

    def getX(self):
        return self.x

    def getY(self):
        return self.y

    def dist_from(self, other):
        xDist = self.x - other.x
        yDist = self.y - other.y
        return (xDist ** 2 + yDist ** 2) ** 0.5

    def __eq__(self, other):
        return (self.x == other.x and self.y == other.y)

    def __str__(self):
        return '<' + str(self.x) + ',' + str(self.y) + '>'


class Campus(object):
    def __init__(self, center_loc):
        self.center_loc = center_loc

    def __str__(self):
        return str(self.center_loc)


class MITCampus(Campus):
    """ A MITCampus is a Campus that contains tents """
    tents_list = []
    def __init__(self, center_loc, tent_loc=Location(0, 0)):
        """ Assumes center_loc and tent_loc are Location objects
        Initializes a new Campus centered at location center_loc
        with a tent at location tent_loc """
        # Your code here
        Campus.__init__(self, center_loc)
        self.tent_loc = tent_loc
        self.tents_list.append(self.tent_loc)

    def add_tent(self, new_tent_loc):
        """ Assumes new_tent_loc is a Location
        Adds new_tent_loc to the campus only if the tent is at least 0.5 distance
        away from all other tents already there. Campus is unchanged otherwise.
        Returns True if it could add the tent, False otherwise. """
        # Your code here

        new_tent_flag = True
        for loc in self.tents_list:
            if loc == new_tent_loc or new_tent_loc.dist_from(loc) < 0.5:
                new_tent_flag = False

        if new_tent_flag:
            self.tents_list.append(new_tent_loc)
            return True
        else:
            return False


    def get_tents(self):
        """ Returns a list of all tents on the campus. The list should contain
        the string representation of the Location of a tent. The list should
        be sorted by the x coordinate of the location. """
        # Your code here
        new_list_sorted = sorted(self.tents_list, key=lambda tent: tent.getX())
        str_list = []
        for x in new_list_sorted:
            str_list.append(x.__str__())

        return str_list

测试案例：

Test: 0


        c = MITCampus(Location(1,2))
        print(c.add_tent(Location(1,2)))
        print(c.add_tent(Location(0,0)))
        print(c.add_tent(Location(2,3)))
        print(c.add_tent(Location(2,3)))
        print(c.get_tents())


Output:

    True
    False
    True
    False
    ['<0,0>', '<1,2>', '<2,3>']

Test: 1


        init campus with default tent loc
        c = MITCampus(Location(-1,-2))
        print(sorted(c.get_tents()))


Output:

    ['<0,0>','<0,0>', '<1,2>', '<2,3>']

Expected Output:

        ['<0,0>']

可以看出，第二个实例应该覆盖前一个实例，而是添加它。代码中的问题是什么？以及如何解决它？

Answer 1

该对象被覆盖，但您将tents_list定义为类变量，因此MITCampus 的所有实例共享此列表。因此，任何新实例都会添加到该列表中，而您似乎并未“覆盖”。

如果您想要“覆盖”行为，请将tents_list移动到__init__方法self.tents_list。只有这样，每个实例才会是唯一的。

Answer 2

确定，您的tents_list属性是类属性，因此即使c对象被覆盖，tents_list属性也保持不变。最好将tents_list设为对象参数，以便tents_list属性也被覆盖。

def __init__(self, center_loc, tent_loc=Location(0, 0)):
    Campus.__init__(self, center_loc)
    self.tents_list = []  # <--- Add this
    self.tent_loc = tent_loc
    self.tents_list.append(self.tent_loc)

Python OOP，无法覆盖实例

2 个答案: