Silhouette should return None (#1770)

Author: MaxHalford

river/metrics/silhouette.py

@@ -70,7 +70,7 @@ def _find_distance_second_closest_center(centers, x):
 
     def update(self, x, y_pred, centers, w=1.0):
         if y_pred not in centers or len(centers) < 2:
-            return self
+            return
 
         distance_closest_centroid = utils.math.minkowski_distance(centers[y_pred], x, 2)
         self._sum_distance_closest_centroid += distance_closest_centroid
@@ -80,7 +80,7 @@ def update(self, x, y_pred, centers, w=1.0):
 
     def revert(self, x, y_pred, centers, w=1.0):
         if y_pred not in centers or len(centers) < 2:
-            return self
+            return
 
         distance_closest_centroid = utils.math.minkowski_distance(centers[y_pred], x, 2)
         self._sum_distance_closest_centroid -= distance_closest_centroid