Fix/checkpoint no file error (#541)

* Fix bug where checkpointers would error if they couldn't find the previous checkpoint file

* Update changelog

* Fix python2 test

CHANGELOG.md

@@ -28,6 +28,7 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed a bug with the once and once_per_epoch decorators
 - Fixed a bug where the test criterion wouldn't accept a function of state
 - Fixed a bug where type inference would not work correctly when chaining ``Trial`` methods
+- Fixed a bug where checkpointers would error when they couldn't find the old checkpoint to overwrite
 
 ## [0.3.0] - 2019-02-28
 ### Added

tests/callbacks/test_checkpointers.py

@@ -4,14 +4,25 @@
 import torchbearer
 from torchbearer import Trial
 from torchbearer.callbacks.checkpointers import _Checkpointer, ModelCheckpoint, MostRecent, Interval, Best
-
+import warnings
 
 class TestCheckpointer(TestCase):
     @patch('os.makedirs')
     def test_make_dirs(self, mock_dirs):
         _Checkpointer('thisdirectoryshouldntexist/norshouldthis/model.pt')
         mock_dirs.assert_called_once_with('thisdirectoryshouldntexist/norshouldthis')
 
+    @patch('torch.save')
+    @patch('os.makedirs')
+    def test_no_existing_file(self, mock_dirs, mock_save):
+        check = _Checkpointer('thisdirectoryshouldntexist/norshouldthis/model.pt')
+        check.most_recent = 'thisfiledoesnotexist.pt'
+        with warnings.catch_warnings(record=True) as w:
+            warnings.simplefilter('always')
+            check.save_checkpoint({torchbearer.METRICS: {}, torchbearer.SELF: Mock()}, True)
+            self.assertTrue(len(w) == 1)
+            self.assertTrue('Failed to delete old file' in str(w[-1].message))
+
     @patch("torch.save")
     def test_save_checkpoint_save_filename(self, mock_save):
         torchmodel = Mock()

torchbearer/callbacks/checkpointers.py

@@ -4,6 +4,7 @@
 
 from torchbearer.callbacks.callbacks import Callback
 import os
+import warnings
 
 
 class _Checkpointer(Callback):
@@ -30,7 +31,10 @@ def save_checkpoint(self, model_state, overwrite_most_recent=False):
         filepath = self.fileformat.format(**string_state)
 
         if self.most_recent is not None and overwrite_most_recent:
-            os.remove(self.most_recent)
+            try:
+                os.remove(self.most_recent)
+            except OSError:
+                warnings.warn('Failed to delete old file. Are you running two checkpointers with the same filename?')
 
         if self.save_model_params_only:
             torch.save(model_state[torchbearer.MODEL].state_dict(), filepath, pickle_module=self.pickle_module,