fix behavior when num_serialized_models_to_keep is 0 (#2880)

* fix behavior when num_serialized_models_to_keep is 0

* pylint

allennlp/tests/training/checkpointer_test.py

@@ -48,6 +48,18 @@ def test_default(self):
         models, training = self.retrieve_and_delete_saved()
         assert models == training == target
 
+    def test_keep_zero(self):
+        checkpointer = Checkpointer(serialization_dir=self.TEST_DIR,
+                                    num_serialized_models_to_keep=0)
+        for e in range(10):
+            checkpointer.save_checkpoint(epoch=e,
+                                         model_state={"epoch": e},
+                                         training_states={"epoch": e},
+                                         is_best_so_far=True)
+        files = os.listdir(self.TEST_DIR)
+        assert 'model_state_epoch_1.th' not in files
+        assert 'training_state_epoch_1.th' not in files
+
     def test_with_time(self):
         """
         Tests that keep_serialized_model_every_num_seconds parameter causes a checkpoint to be saved

allennlp/training/checkpointer.py

@@ -49,7 +49,7 @@ def save_checkpoint(self,
                             "Copying weights to '%s/best.th'.", self._serialization_dir)
                 shutil.copyfile(model_path, os.path.join(self._serialization_dir, "best.th"))
 
-            if self._num_serialized_models_to_keep and self._num_serialized_models_to_keep >= 0:
+            if self._num_serialized_models_to_keep is not None and self._num_serialized_models_to_keep >= 0:
                 self._serialized_paths.append((time.time(), model_path, training_path))
                 if len(self._serialized_paths) > self._num_serialized_models_to_keep:
                     paths_to_remove = self._serialized_paths.pop(0)