diff --git a/docs/ml/3.决策树.md b/docs/ml/3.决策树.md
index 18af0f79..49f56580 100644
--- a/docs/ml/3.决策树.md
+++ b/docs/ml/3.决策树.md
@@ -318,7 +318,7 @@ def classify(inputTree, featLabels, testVec):
         classLabel 分类的结果值，需要映射label才能知道名称
     """
     # 获取tree的根节点对于的key值
-    firstStr = inputTree.keys()[0]
+    firstStr = list(inputTree.keys())[0]
     # 通过key得到根节点对应的value
     secondDict = inputTree[firstStr]
     # 判断根节点名称获取根节点在label中的先后顺序，这样就知道输入的testVec怎么开始对照树来做分类