Warn user if storage file is missing

Also warn user if the given filename is a directory.

oauth2client/file.py

@@ -20,17 +20,28 @@
 
 import os
 import threading
+import warnings
 
 from oauth2client import client
 
 
 __author__ = 'jcgregorio@google.com (Joe Gregorio)'
 
+_SYM_LINK_MESSAGE = 'File: {0}: Is a symbolic link.'
+
+_IS_DIR_MESSAGE = '{0}: Is a directory'
+
+_MISSING_FILE_MESSAGE = 'Cannot access {0}: No such file or directory'
+
 
 class CredentialsFileSymbolicLinkError(Exception):
     """Credentials files must not be symbolic links."""
 
 
+class CredentialsFileDirectoryError(Exception):
+    """Credentials files must regular files, not directories."""
+
+
 class Storage(client.Storage):
     """Store and retrieve a single credential to and from a file."""
 
@@ -41,7 +52,12 @@ def __init__(self, filename):
     def _validate_file(self):
         if os.path.islink(self._filename):
             raise CredentialsFileSymbolicLinkError(
-                'File: {0} is a symbolic link.'.format(self._filename))
+                _SYM_LINK_MESSAGE.format(self._filename))
+        elif os.path.isdir(self._filename):
+            raise CredentialsFileDirectoryError(
+                _IS_DIR_MESSAGE.format(self._filename))
+        elif not os.path.isfile(self._filename):
+            warnings.warn(_MISSING_FILE_MESSAGE.format(self._filename))
 
     def locked_get(self):
         """Retrieve Credential from file.

tests/test_file.py

@@ -24,7 +24,9 @@
 import pickle
 import stat
 import tempfile
+import warnings
 
+import mock
 import six
 from six.moves import http_client
 import unittest2
@@ -54,6 +56,7 @@ def tearDown(self):
             pass
 
     def setUp(self):
+        warnings.simplefilter("ignore")
         try:
             os.unlink(FILENAME)
         except OSError:
@@ -74,11 +77,23 @@ def _create_test_credentials(self, client_id='some_client_id',
             user_agent)
         return credentials
 
-    def test_non_existent_file_storage(self):
+    @mock.patch('warnings.warn')
+    def test_non_existent_file_storage(self, warn_mock):
         s = file.Storage(FILENAME)
         credentials = s.get()
+        warn_mock.assert_called_with(
+            file._MISSING_FILE_MESSAGE.format(FILENAME))
         self.assertEquals(None, credentials)
 
+    def test_directory_file_storage(self):
+        s = file.Storage(FILENAME)
+        os.mkdir(FILENAME)
+        try:
+            with self.assertRaises(file.CredentialsFileDirectoryError):
+                s.get()
+        finally:
+            os.rmdir(FILENAME)
+
     @unittest2.skipIf(not hasattr(os, 'symlink'), 'No symlink available')
     def test_no_sym_link_credentials(self):
         SYMFILENAME = FILENAME + '.sym'