Fix a crash in unnecessary-dict-index-lookup when subscripting an attribute (#6579)

Author: jacobtylerwalls

ChangeLog

@@ -29,6 +29,10 @@ Release date: TBA
 
  Relates to #6531
 
+* Fix a crash in ``unnecessary-dict-index-lookup`` when subscripting an attribute.
+
+  Closes #6557
+
 * Fix a crash when accessing ``__code__`` and assigning it to a variable.
 
   Closes #6539

doc/whatsnew/2.13.rst

@@ -653,3 +653,7 @@ Other Changes
 * Fix ``IndexError`` crash in ``uninferable_final_decorators`` method.
 
  Relates to #6531
+
+* Fix a crash in ``unnecessary-dict-index-lookup`` when subscripting an attribute.
+
+  Closes #6557

pylint/checkers/refactoring/refactoring_checker.py

@@ -1938,6 +1938,7 @@ def _check_unnecessary_dict_index_lookup(
                     elif isinstance(value, nodes.Subscript):
                         if (
                             not isinstance(node.target, nodes.AssignName)
+                            or not isinstance(value.value, nodes.Name)
                             or node.target.name != value.value.name
                             or iterating_object_name != subscript.value.as_string()
                         ):

tests/functional/u/unnecessary/unnecessary_dict_index_lookup.py

@@ -112,3 +112,10 @@ class Foo:
 d = {}
 for key, in d.items():
     print(d[key])
+
+# Test subscripting an attribute
+# https://github.com/PyCQA/pylint/issues/6557
+f = Foo()
+for input_output in d.items():
+    f.input_output = input_output  # pylint: disable=attribute-defined-outside-init
+    print(d[f.input_output[0]])