python - 如何从父类扩展列表？

Question

我有以下代码。父类有一个项目列表，子级需要向其中添加。

parent 的每个实例都需要有这个列表，每个 child 都需要有那个列表 + 额外的值。

class Parent(object):
    a_list = ['parent_item1', 'parent_item2', ]

    def print_list(self):
        print(self.a_list)


class Child1(Parent):
    def __init__(self, *args, **kwargs):
        super(Child1, self).__init__(*args, **kwargs)
        self.a_list += ['child1_item']


class Child2(Parent):
    def __init__(self, *args, **kwargs):
        super(Child2, self).__init__(*args, **kwargs)
        self.a_list += ['child2_item']


parent = Parent()
child1 = Child1()
child2 = Child2()

parent.print_list()
    # >> ['parent_item1', 'parent_item2', 'child1_item', 'child2_item']
child1.print_list()
    # >> ['parent_item1', 'parent_item2', 'child1_item', 'child2_item']
child2.print_list()
    # >> ['parent_item1', 'parent_item2', 'child1_item', 'child2_item']

我怎样才能得到下面的结果？

['parent_item1', 'parent_item2', ]
['parent_item1', 'parent_item2', 'child1_item', ]
['parent_item1', 'parent_item2', 'child2_item', ]

Answer 1

list += other_list 就地扩展列表。使用返回新列表的 + 运算符:

class Parent(object):
    a_list = ['parent_item1', 'parent_item2', ]
    def print_list(self):
        print(self.a_list)

class Child1(Parent):
    def __init__(self, *args, **kwargs):
        super(Child1, self).__init__(*args, **kwargs)
        self.a_list = self.a_list + ['child1_item']  # <-------

class Child2(Parent):
    def __init__(self, *args, **kwargs):
        super(Child2, self).__init__(*args, **kwargs)
        self.a_list = self.a_list + ['child2_item']  # <-------


parent = Parent()
child1 = Child1()
child2 = Child2()
parent.print_list()
child1.print_list()
child2.print_list()

输出:

['parent_item1', 'parent_item2']
['parent_item1', 'parent_item2', 'child1_item']
['parent_item1', 'parent_item2', 'child2_item']

---

或者，您可以复制父类的列表:

...
self.a_list = self.a_list[:]
self.a_list += ['child1_item']