Python类别树/叶类保持变量

Question

对于Category/Leaf实现，我有以下类：

class CategoryMapper():
    @staticmethod
    def get_tree():
        categories = []
        tree_categories = Category.objects.filter(parent_id__isnull = True) 
        for tree_category in tree_categories:
            leaf_categories = CategoryMapper.get_leafs(tree_category)
            categories += leaf_categories

        return categories

    @staticmethod
    def get_leafs(tree_category, leaf_categories = [], depth = 0):
        if depth > 0:
            child_categories = Category.objects.filter(parent_id__exact = tree_category.id)
            if len(child_categories):
                depth += 1
                for sub_category in child_categories:
                    sub_category_name = ((depth - 1) * '-') + sub_category.category
                    leaf_categories.append([sub_category.id, sub_category_name])

                    return CategoryMapper.get_leafs(sub_category, leaf_categories, depth)
            else:
                return leaf_categories
        else:
            leaf_categories.append([tree_category.id, tree_category.category])
            depth += 1

            return CategoryMapper.get_leafs(tree_category, leaf_categories, depth)

我有以下日期： ID |类别|父ID 1 |测试1 |无 2 |测试1儿童| 1 无测试

当我运行它的时候(CategoryMapper.get_树（））我得到：

^{pr2}$

当我再次运行它时，我得到：

[[1, u'Test 1'], [2, u'-Test 1 Child'], [3, u'Test 2'], [1, u'Test 1'], [2, u'-Test 1 Child'], [1, u'Test 1'], [2, u'-Test 1 Child'], [3, u'Test 2'], [1, u'Test 1'], [2, u'-Test 1 Child'], [3, u'Test 2']]

就像get_tree（）中的categories变量在遍历tree\u categories时，每次运行for循环时都会保持其状态。我做错什么了？为什么要保持这样的状态？在

Answer 1

啊，你需要避免使用可变对象作为函数的默认参数，比如[]。相反，只需如下定义get_leafs：

def get_leafs(tree_category, leaf_categories = None, depth = 0):
    if leaf_categories is None:
        leaf_categories = []
    …