diff --git a/tests/functional/thermal_heuristic/test_thermal_heuristic_six_clusters.py b/tests/functional/thermal_heuristic/test_thermal_heuristic_six_clusters.py
index ef77808..f794d10 100644
--- a/tests/functional/thermal_heuristic/test_thermal_heuristic_six_clusters.py
+++ b/tests/functional/thermal_heuristic/test_thermal_heuristic_six_clusters.py
@@ -77,7 +77,7 @@ def test_accurate_heuristic(
     )
 
     for j, cluster in enumerate(heuristic_components):
-        input_data = np.loadtxt(data_path / "accurate/itr1_accurate_cluster{j+1}.txt")
+        input_data = np.loadtxt(data_path / f"accurate/itr1_accurate_cluster{j+1}.txt")
         nb_on_1 = pd.DataFrame(
             np.transpose(np.ceil(np.round(input_data, 12))),
             index=list(range(number_hours)),
@@ -110,7 +110,7 @@ def test_accurate_heuristic(
         )
 
         expected_output = np.loadtxt(
-            data_path / "accurate/itr2_accurate_cluster{j+1}.txt"
+            data_path / f"accurate/itr2_accurate_cluster{j+1}.txt"
         )
         assert list(nb_on_heuristic[:, 0]) == [
             pytest.approx(x) for x in expected_output
@@ -152,7 +152,7 @@ def test_fast_heuristic(
         pmax = thermal_problem_builder.database.get_value(
             ComponentParameterIndex(cluster, "p_max"), 0, 0
         )
-        input_data = np.loadtxt(data_path / "fast/itr1_fast_cluster{j+1}.txt")
+        input_data = np.loadtxt(data_path / f"fast/itr1_fast_cluster{j+1}.txt")
 
         nb_on_1 = pd.DataFrame(
             np.ceil(np.round(input_data / pmax, 12)),  # type: ignore
@@ -188,7 +188,7 @@ def test_fast_heuristic(
             param_needed_to_compute=["p_min", "max_generating"],
         )
 
-        expected_output = np.loadtxt(data_path / "fast/itr2_fast_cluster{j+1}.txt")
+        expected_output = np.loadtxt(data_path / f"fast/itr2_fast_cluster{j+1}.txt")
         for t in range(number_hours):
             assert thermal_problem_builder.database.get_value(
                 ComponentParameterIndex(cluster, "min_generating"),