Add an exception for `IndexError` inside `uninferable_final_decorator` (#6532)

Co-authored-by: Pierre Sassoulas <pierre.sassoulas@gmail.com>
Co-authored-by: Daniël van Noord <13665637+DanielNoord@users.noreply.github.com>

4 files changed, 55 insertions, 12 deletions

diff --git a/ChangeLog b/ChangeLog
index 7d8d60e85..07bf60acb 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -20,6 +20,9 @@ What's New in Pylint 2.13.9?
 ============================
 Release date: TBA
 
+* Fix ``IndexError`` crash in ``uninferable_final_decorators`` method.
+
+ Relates to #6531
 
 
 What's New in Pylint 2.13.8?
diff --git a/doc/whatsnew/2.13.rst b/doc/whatsnew/2.13.rst
index 56914559f..e851af65d 100644
--- a/doc/whatsnew/2.13.rst
+++ b/doc/whatsnew/2.13.rst
@@ -639,3 +639,7 @@ Other Changes
   ``open``
 
   Closes #6414
+
+* Fix ``IndexError`` crash in ``uninferable_final_decorators`` method.
+
+ Relates to #6531
diff --git a/pylint/checkers/utils.py b/pylint/checkers/utils.py
index ec5f2cddc..9cba6e057 100644
--- a/pylint/checkers/utils.py
+++ b/pylint/checkers/utils.py
@@ -820,28 +820,34 @@ def uninferable_final_decorators(
     """
     decorators = []
     for decorator in getattr(node, "nodes", []):
+        import_nodes: tuple[nodes.Import | nodes.ImportFrom] | None = None
+
+        # Get the `Import` node. The decorator is of the form: @module.name
         if isinstance(decorator, nodes.Attribute):
-            try:
-                import_node = decorator.expr.lookup(decorator.expr.name)[1][0]
-            except AttributeError:
-                continue
+            inferred = safe_infer(decorator.expr)
+            if isinstance(inferred, nodes.Module) and inferred.qname() == "typing":
+                _, import_nodes = decorator.expr.lookup(decorator.expr.name)
+
+        # Get the `ImportFrom` node. The decorator is of the form: @name
         elif isinstance(decorator, nodes.Name):
-            lookup_values = decorator.lookup(decorator.name)
-            if lookup_values[1]:
-                import_node = lookup_values[1][0]
-            else:
-                continue  # pragma: no cover # Covered on Python < 3.8
-        else:
+            _, import_nodes = decorator.lookup(decorator.name)
+
+        # The `final` decorator is expected to be found in the
+        # import_nodes. Continue if we don't find any `Import` or `ImportFrom`
+        # nodes for this decorator.
+        if not import_nodes:
             continue
+        import_node = import_nodes[0]
 
         if not isinstance(import_node, (astroid.Import, astroid.ImportFrom)):
             continue
 
         import_names = dict(import_node.names)
 
-        # from typing import final
+        # Check if the import is of the form: `from typing import final`
         is_from_import = ("final" in import_names) and import_node.modname == "typing"
-        # import typing
+
+        # Check if the import is of the form: `import typing`
         is_import = ("typing" in import_names) and getattr(
             decorator, "attrname", None
         ) == "final"
diff --git a/tests/functional/r/regression/regression_6531_crash_index_error.py b/tests/functional/r/regression/regression_6531_crash_index_error.py
new file mode 100644
index 000000000..6cdc96617
--- /dev/null
+++ b/tests/functional/r/regression/regression_6531_crash_index_error.py
@@ -0,0 +1,30 @@
+"""Regression test for https://github.com/PyCQA/pylint/issues/6531."""
+
+# pylint: disable=missing-docstring, redefined-outer-name
+
+import pytest
+
+
+class Wallet:
+    def __init__(self):
+        self.balance = 0
+
+    def add_cash(self, earned):
+        self.balance += earned
+
+    def spend_cash(self, spent):
+        self.balance -= spent
+
+@pytest.fixture
+def my_wallet():
+    '''Returns a Wallet instance with a zero balance'''
+    return Wallet()
+
+@pytest.mark.parametrize("earned,spent,expected", [
+    (30, 10, 20),
+    (20, 2, 18),
+])
+def test_transactions(my_wallet, earned, spent, expected):
+    my_wallet.add_cash(earned)
+    my_wallet.spend_cash(spent)
+    assert my_wallet.balance == expected