Skip unnecessary-list-index-lookup if we encounter if statements (#7334)

Author: DanielNoord

doc/whatsnew/fragments/6896.false_positive

@@ -0,0 +1,3 @@
+``unnecessary-list-index-lookup`` is now more conservative to avoid potential false positives.
+
+Closes #6896

pylint/checkers/refactoring/refactoring_checker.py

@@ -2127,6 +2127,15 @@ def _check_unnecessary_list_index_lookup(
         )
         has_nested_loops = next(nested_loops, None) is not None
 
+        # Check if there are any if statements within the loop in question;
+        # If so, we will be more conservative about reporting errors as we
+        # can't yet do proper control flow analysis to be sure when
+        # reassignment will affect us
+        if_statements = itertools.chain.from_iterable(
+            child.nodes_of_class(nodes.If) for child in children
+        )
+        has_if_statements = next(if_statements, None) is not None
+
         for child in children:
             for subscript in child.nodes_of_class(nodes.Subscript):
                 if isinstance(node, nodes.For) and _is_part_of_assignment_target(
@@ -2170,6 +2179,8 @@ def _check_unnecessary_list_index_lookup(
                         # loops we don't want to report this unless we get to the
                         # end of the loop without updating the collection
                         bad_nodes.append(subscript)
+                    elif has_if_statements:
+                        continue
                     else:
                         self.add_message(
                             "unnecessary-list-index-lookup",

tests/functional/u/unnecessary/unnecessary_list_index_lookup.py

@@ -74,3 +74,10 @@ def process_list_again(data):
     print(updated_list[idx]) # [unnecessary-list-index-lookup]
     updated_list[idx] -= 1
     print(updated_list[idx])
+
+# Regression test for https://github.com/PyCQA/pylint/issues/6896
+parts = ["a", "b", "c", "d"]
+for i, part in enumerate(parts):
+    if i == 3:  # more complex condition actually
+        parts.insert(i, "X")
+    print(part, parts[i])