fix pre 3.8 compatible of list[str]

Author: caesar0301

.github/workflows/python-package.yml

@@ -15,7 +15,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.6", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
+        python-version: ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13"]
 
     steps:
     - uses: actions/checkout@v4

treelib/node.py

@@ -27,13 +27,19 @@
 
 import copy
 import uuid
+import sys
 from collections import defaultdict
 from warnings import warn
-from typing import cast, Any, Optional, Union
+from typing import cast, List, Any, Optional, Union
 
 from .exceptions import NodePropertyError
 from .misc import deprecated
 
+if sys.version_info >= (3, 9):
+    StrList = list[str]
+else:
+    StrList = List[str]  # Python 3.8 and earlier
+
 
 class Node(object):
     """
@@ -143,7 +149,7 @@ def set_predecessor(self, nid: Optional[str], tree_id: Optional[str]) -> None:
         """Set the value of `_predecessor`."""
         self._predecessor[tree_id] = nid
 
-    def successors(self, tree_id: Optional[str]):
+    def successors(self, tree_id: Optional[str]) -> StrList:
         """
         With a getting operator, a list of IDs of node's children is obtained. With
         a setting operator, the value can be list, set, or dict. For list or set,

treelib/tree.py

@@ -37,9 +37,17 @@
 import codecs
 import json
 import uuid
+import sys
 from copy import deepcopy
 from six import python_2_unicode_compatible, iteritems
-from typing import cast, Any, Callable, Optional, Union
+from typing import cast, List, Any, Callable, Optional, Union
+
+if sys.version_info >= (3, 9):
+    StrList = list[str]
+    StrLList = list[list[str]]
+else:
+    StrList = List[str]  # Python 3.8 and earlier
+    StrLList = List[List[str]]
 
 try:
     from StringIO import StringIO  # type: ignore
@@ -275,7 +283,7 @@ def __get_iter(
         filter_: Callable[[Node], bool],
         key: Optional[Callable[[Node], Node]],
         reverse: bool,
-        dt,
+        dt,  # tuple[str, str, str]
         is_last: list,
         sorting: bool,
     ):
@@ -742,7 +750,7 @@ def paste(self, nid: str, new_tree, deep: bool = False):
         self.__update_bpointer(new_tree.root, nid)
         self.__update_fpointer(nid, new_tree.root, self.node_class.ADD)
 
-    def paths_to_leaves(self):
+    def paths_to_leaves(self) -> StrLList:
         """
         Use this function to get the identifiers allowing to go from the root
         nodes to each leaf.