make a backup of the data file before saving

Right now, if a program crashes in the middle of saving, you lose
all your data. This ensures that the old file is first moved, then
the new file is saved, and only then the old file is removed.

Author: basnijholt

adaptive/learner/base_learner.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 
 import abc
+import os
 from contextlib import suppress
 from copy import deepcopy
 
@@ -174,8 +175,16 @@ def save(self, fname, compress=True):
             using compression, one must load it with compression too.
         """
         data = self._get_data()
+
+        backup = os.path.join(".", fname)
+        if os.path.exists(fname):
+            os.rename(backup)
+
         save(fname, data, compress)
 
+        if os.path.exists(backup):
+            os.remove(backup)
+
     def load(self, fname, compress=True):
         """Load the data of a learner from a pickle file.
 
@@ -187,6 +196,12 @@ def load(self, fname, compress=True):
             If the data is compressed when saved, one must load it
             with compression too.
         """
+        with suppress(FileNotFoundError):
+            backup = os.path.join(".", fname)
+            if os.path.getctime(fname) < os.path.getctime(backup):
+                # the backup is the newer file
+                fname = backup
+
         with suppress(FileNotFoundError, EOFError):
             data = load(fname, compress)
             self._set_data(data)