Don't emit modified-iterating-dict when updating existing keys (#7037)

Co-authored-by: Pierre Sassoulas <[email protected]>

doc/whatsnew/2/2.14/full.rst

@@ -13,11 +13,16 @@ Release date: TBA
 
   Closes #7006
 
+* Fixed a false positive for ``modified-iterating-dict`` when updating an existing key.
+
+  Closes #6179
+
 * Fixed an issue where many-core Windows machines (>~60 logical processors) would hang when
   using the default jobs count.
 
   Closes #6965
 
+
 What's New in Pylint 2.14.3?
 ----------------------------
 Release date: 2022-06-18

pylint/checkers/modified_iterating_checker.py

@@ -130,6 +130,15 @@ def _modified_iterating_dict_cond(
     ) -> bool:
         if not self._is_node_assigns_subscript_name(node):
             return False
+        # Do not emit when merely updating the same key being iterated
+        if (
+            isinstance(iter_obj, nodes.Name)
+            and iter_obj.name == node.targets[0].value.name
+            and isinstance(iter_obj.parent.target, nodes.AssignName)
+            and isinstance(node.targets[0].slice, nodes.Name)
+            and iter_obj.parent.target.name == node.targets[0].slice.name
+        ):
+            return False
         infer_val = utils.safe_infer(node.targets[0].value)
         if not isinstance(infer_val, nodes.Dict):
             return False

tests/functional/m/modified_iterating.py

@@ -71,3 +71,13 @@ def format_manifest_serializer_errors(errors):
 dict2 = {"2": 2}
 for item in dict1:
     dict2[item] = 1
+
+
+def update_existing_key():
+    """No message when updating existing keys"""
+    for key in my_dict:
+        my_dict[key] = 1
+
+    for key in my_dict:
+        new_key = key.lower()
+        my_dict[new_key] = 1  # [modified-iterating-dict]

tests/functional/m/modified_iterating.txt

@@ -8,3 +8,4 @@ modified-iterating-list:48:4:48:23::Iterated list 'item_list' is being modified
 modified-iterating-list:52:4:52:23::Iterated list 'item_list' is being modified inside for loop body, consider iterating through a copy of it instead.:INFERENCE
 modified-iterating-list:55:12:55:31::Iterated list 'item_list' is being modified inside for loop body, consider iterating through a copy of it instead.:INFERENCE
 modified-iterating-list:57:16:57:35::Iterated list 'item_list' is being modified inside for loop body, consider iterating through a copy of it instead.:INFERENCE
+modified-iterating-dict:83:8:83:28:update_existing_key:Iterated dict 'my_dict' is being modified inside for loop body, iterate through a copy of it instead.:INFERENCE