Update flow_statistics.py

src/lbmpy/flow_statistics.py

@@ -70,7 +70,8 @@ def welford_assignments(field, mean_field, sum_of_products_field=None):
     if sum_of_products_field is not None:
         delta2 = sp.symbols(f"delta2_:{dim}")
         for i in range(dim):
-            main_assignments.append(ps.Assignment(delta2[i], welford_field.at_index(i) - welford_mean_field.at_index(i)))
+            main_assignments.append(
+                ps.Assignment(delta2[i], welford_field.at_index(i) - welford_mean_field.at_index(i)))
             for j in range(dim):
                 idx = i * dim + j
                 main_assignments.append(ps.Assignment(