Python的类别树/叶类保持变量State

Question

我有一个类别/叶实现以下类：

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 |分类|父ID 1 |测试1 |无 2 |测试和1名儿童| 1 3 |测试2 |无

当我运行它（CategoryMapper.get_tree（））我得到：

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

当我再次运行它，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']] 

这就像在get_tree的类别变量（）是有它的状态保持每次通过tree_categories去当for循环跑。我究竟做错了什么？为什么它保持这样的状态？

Answer 1

啊，你需要避免使用可变对象作为函数的默认参数，如[]。相反，只需定义get_leafs像这样：

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