Fix Python IndexError of case4: paddle.optimizer.lr.PiecewiseDecay (#49987)

python/paddle/fluid/tests/unittests/test_lr_scheduler.py

@@ -728,6 +728,17 @@ class TestLRScheduler(unittest.TestCase):
                 step_size_down=-1,
                 scale_mode='test',
             )
+        # check empty boundaries
+        with self.assertRaises(ValueError):
+            paddle.optimizer.lr.PiecewiseDecay(boundaries=[], values=[])
+        # check non-empty boundaries but empty values
+        with self.assertRaises(ValueError):
+            paddle.optimizer.lr.PiecewiseDecay(boundaries=[100, 200], values=[])
+        # check boundaries and values has same length
+        with self.assertRaises(ValueError):
+            paddle.optimizer.lr.PiecewiseDecay(
+                boundaries=[100, 200], values=[0.5, 0.1]
+            )
 
         func_api_kwargs = [
             (

python/paddle/optimizer/lr.py

@@ -391,6 +391,14 @@ class PiecewiseDecay(LRScheduler):
     """
 
     def __init__(self, boundaries, values, last_epoch=-1, verbose=False):
+        if len(boundaries) == 0:
+            raise ValueError('The boundaries cannot be empty.')
+
+        if len(values) <= len(boundaries):
+            raise ValueError(
+                f'The values have one more element than boundaries, but received len(values) [{len(values)}] < len(boundaries) + 1 [{len(boundaries) + 1}].'
+            )
+
         self.boundaries = boundaries
         self.values = values
         super().__init__(last_epoch=last_epoch, verbose=verbose)

python/paddle/tests/test_callback_reduce_lr_on_plateau.py

@@ -88,7 +88,7 @@ class TestReduceLROnPlateau(unittest.TestCase):
 
         optim = paddle.optimizer.Adam(
             learning_rate=paddle.optimizer.lr.PiecewiseDecay(
-                [0.001, 0.0001], [5, 10]
+                [0.001, 0.0001], [5, 10, 10]
             ),
             parameters=net.parameters(),
         )