Type annotate _preorder_depth_first_walk

Improve type annotations.

Signed-off-by: Teodora Sechkova <tsechkova@vmware.com>

Author: sechkova

tuf/ngclient/updater.py

@@ -6,12 +6,13 @@
 
 import logging
 import os
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List, Optional, Set, Tuple, Union
 from urllib import parse
 
 from securesystemslib import util as sslib_util
 
 from tuf import exceptions
+from tuf.api.metadata import Targets
 from tuf.ngclient._internal import requests_fetcher, trusted_metadata_set
 from tuf.ngclient.config import UpdaterConfig
 from tuf.ngclient.fetcher import FetcherInterface
@@ -89,7 +90,9 @@ def refresh(self) -> None:
         self._load_snapshot()
         self._load_targets("targets", "root")
 
-    def get_one_valid_targetinfo(self, target_path: str) -> Dict:
+    def get_one_valid_targetinfo(
+        self, target_path: str
+    ) -> Union[Dict[str, Any], None]:
         """
         Returns the target information for a target identified by target_path.
 
@@ -303,15 +306,17 @@ def _load_targets(self, role: str, parent_role: str) -> None:
             self._trusted_set.update_delegated_targets(data, role, parent_role)
             self._persist_metadata(role, data)
 
-    def _preorder_depth_first_walk(self, target_filepath: str) -> Dict:
+    def _preorder_depth_first_walk(
+        self, target_filepath: str
+    ) -> Union[Dict[str, Any], None]:
         """
         Interrogates the tree of target delegations in order of appearance
         (which implicitly order trustworthiness), and returns the matching
         target found in the most trusted role.
         """
 
         role_names = [("targets", "root")]
-        visited_role_names = set()
+        visited_role_names: Set[Tuple[str, str]] = set()
         number_of_delegations = self.config.max_delegations
 
         # Preorder depth-first traversal of the graph of target delegations.
@@ -329,7 +334,7 @@ def _preorder_depth_first_walk(self, target_filepath: str) -> Dict:
             # its targets, delegations, and child roles can be inspected.
             self._load_targets(role_name, parent_role)
 
-            role_metadata = self._trusted_set[role_name].signed
+            role_metadata: Targets = self._trusted_set[role_name].signed
             target = role_metadata.targets.get(target_filepath)
 
             if target is not None: