为什么我的课程实例共享属性？

Question

为什么我的class属性高度对于不同的实例返回相同的值？

th.mat-header-cell,
td.mat-cell {
  text-align: center !important;
}

当我尝试将值添加到高度列表时，另一个实例也将添加该值。我可以通过以下方法解决此问题：

class Tree:
    def __init__(self, age, tree_type, heights=[]):
        self.age = age
        self.tree_type = tree_type.title()
        self.heights = heights

    def __str__(self):
        return (f"I am {self.age} and {self.tree_type}. My heights are: {' '.join(self.heights)}")

    def add_height(self,height):
        self.heights += [height]
        self.age += 1
        print(f"Added height of {self.heights[-1]} to {self.tree_type} tree.")

oak_tree = Tree(1, 'oak')
birch_tree = Tree(2, 'Birch')

print(oak_tree)
print(birch_tree)
oak_tree.add_height('5m')
birch_tree.add_height('10m')
print(oak_tree)
print(birch_tree)

#output
I am 1 and Oak. My heights are:
I am enter code here2 and Birch. My heights are: 
Added height of 5m to Oak tree.
Added height of 10m to Birch tree.
I am 2 and Oak. My heights are: 5m 10m
I am 3 and Birch. My heights are: 5m 10m

但是我还是很困惑。我认为，如果没有传递任何值，则可以在属性后使用=符号声明变量。我不能使用列表吗？