diff --git a/recommenders/datasets/amazon_reviews.py b/recommenders/datasets/amazon_reviews.py
index 9107e749e7..a0b34a7439 100644
--- a/recommenders/datasets/amazon_reviews.py
+++ b/recommenders/datasets/amazon_reviews.py
@@ -285,15 +285,15 @@ def _data_generating_no_history_expanding(
             fo = f_test
         if user_id != last_user_id or tfile == "valid" or tfile == "test":
             if last_user_id is not None:
-                history_clk_num = len(movie_id_list)
+                history_clk_num = len(movie_id_list)  # noqa: F821 undefined name 'movie_id_list'
                 cat_str = ""
                 mid_str = ""
                 dt_str = ""
-                for c1 in cate_list[:-1]:
+                for c1 in cate_list[:-1]:  # noqa: F821 undefined name 'cate_list'
                     cat_str += c1 + ","
-                for mid in movie_id_list[:-1]:
+                for mid in movie_id_list[:-1]:  # noqa: F821 undefined name 'movie_id_list'
                     mid_str += mid + ","
-                for dt_time in dt_list[:-1]:
+                for dt_time in dt_list[:-1]:  # noqa: F821 undefined name 'dt_list'
                     dt_str += dt_time + ","
                 if len(cat_str) > 0:
                     cat_str = cat_str[:-1]
diff --git a/recommenders/tuning/nni/ncf_utils.py b/recommenders/tuning/nni/ncf_utils.py
index 3ffa31b5ff..708c1e901c 100644
--- a/recommenders/tuning/nni/ncf_utils.py
+++ b/recommenders/tuning/nni/ncf_utils.py
@@ -47,7 +47,7 @@ def compute_test_results(model, train, test, rating_metrics, ranking_metrics):
     all_predictions = merged[merged.rating.isnull()].drop('rating', axis=1)
 
     for metric in ranking_metrics:
-        test_results[metric] = eval(metric)(test, all_predictions, col_prediction='prediction', k=K)
+        test_results[metric] = eval(metric)(test, all_predictions, col_prediction='prediction', k=K)  # noqa: F821 undefined name 'K'
 
     return test_results
 
diff --git a/tox.ini b/tox.ini
index 514a18b4f1..2fa2b50971 100644
--- a/tox.ini
+++ b/tox.ini
@@ -94,7 +94,7 @@ ignore =
     ; W605  # invalid escape sequence (done)
     ; F403  # 'from X import *' used; unable to detect undefined names (done)
     ; F405  # 'Dense' may be undefined, or defined from star imports: tensorflow.keras.layers (done)
-    F821  # undefined name 'K'
+    ; F821  # undefined name 'K'
 
 # File-specific flake8 ignore rules (too risky to implement the changes)
 per-file-ignores =