Make callback string formatting failure error message more user-friendly.

PiperOrigin-RevId: 284224536
Change-Id: I57b8c2ac506deaaa45365ddb80f564b787abb23f

tensorflow/python/keras/callbacks.py

@@ -1052,7 +1052,14 @@ class ModelCheckpoint(Callback):
     # pylint: disable=protected-access
     if not self.model._in_multi_worker_mode(
     ) or multi_worker_util.should_save_checkpoint():
-      return self.filepath.format(epoch=epoch + 1, **logs)
+      try:
+        # `filepath` may contain placeholders such as `{epoch:02d}` and
+        # `{mape:.2f}`. A mismatch between logged metrics and the path's
+        # placeholders can cause formatting to fail.
+        return self.filepath.format(epoch=epoch + 1, **logs)
+      except KeyError as e:
+        raise KeyError('Failed to format this callback filepath: "{}". '
+                       'Reason: {}'.format(self.filepath, e))
     else:
       # If this is multi-worker training, and this worker should not
       # save checkpoint, we use a temp filepath to store a dummy checkpoint, so

tensorflow/python/keras/callbacks_test.py

@@ -828,6 +828,18 @@ class KerasCallbacksTest(keras_parameterized.TestCase):
                                           'filepath for ModelCheckpoint.'):
       model.fit(train_ds, epochs=1, callbacks=[callback])
 
+  def test_ModelCheckpoint_with_bad_path_placeholders(self):
+    (model, train_ds, callback,
+     filepath) = self._get_dummy_resource_for_model_checkpoint_testing()
+
+    temp_dir = self.get_temp_dir()
+    filepath = os.path.join(temp_dir, 'chkpt_{epoch:02d}_{mape:.2f}.h5')
+    callback = keras.callbacks.ModelCheckpoint(filepath=filepath)
+
+    with self.assertRaisesRegexp(KeyError, 'Failed to format this callback '
+                                           'filepath.*'):
+      model.fit(train_ds, epochs=1, callbacks=[callback])
+
   def test_EarlyStopping(self):
     with self.cached_session():
       np.random.seed(123)