add back the secure write

vcr/files.py

@@ -1,3 +1,4 @@
+import tempfile
 import os
 import yaml
 
@@ -7,13 +8,22 @@ try:
 except ImportError:
     from yaml import Loader, Dumper
 
+def _secure_write(path, contents):
+    """
+    We'll overwrite the old version securely by writing out a temporary
+    version and then moving it to replace the old version
+    """
+    dirname, filename = os.path.split(path)
+    fd, name = tempfile.mkstemp(dir=dirname, prefix=filename)
+    with os.fdopen(fd, 'w') as fout:
+        fout.write(contents)
+        os.rename(name, path)
+
 def load_cassette(cassette_path):
     return yaml.load(open(cassette_path), Loader=Loader)
 
 def save_cassette(cassette_path, data):
-    #TODO: safe overwrite using tmpfile
     dirname, filename = os.path.split(cassette_path)
     if not os.path.exists(dirname):
         os.makedirs(dirname)
-    with open(cassette_path, 'w') as cassette_file:
-        cassette_file.write(yaml.dump(data, Dumper=Dumper))
+    _secure_write(cassette_path, yaml.dump(data, Dumper=Dumper))