Better error handling in Credentials.from_json.

I dropped an over-broad `except` by only catching errors that
`datetime.datetime.strptime` would throw -- forgetting that it might error
differently if one of its arguments were an unexpected type.

Fix and add a test.

Author: craigcitro

oauth2client/client.py

@@ -225,7 +225,8 @@ def _to_json(self, strip):
     for member in strip:
       if member in d:
         del d[member]
-    if 'token_expiry' in d and isinstance(d['token_expiry'], datetime.datetime):
+    if (d.get('token_expiry') and
+        isinstance(d['token_expiry'], datetime.datetime)):
       d['token_expiry'] = d['token_expiry'].strftime(EXPIRY_FORMAT)
     # Add in information we will need later to reconsistitue this instance.
     d['_class'] = t.__name__
@@ -589,7 +590,7 @@ def from_json(cls, s):
       An instance of a Credentials subclass.
     """
     data = json.loads(s)
-    if ('token_expiry' in data and
+    if (data.get('token_expiry') and
         not isinstance(data['token_expiry'], datetime.datetime)):
       try:
         data['token_expiry'] = datetime.datetime.strptime(

tests/test_oauth2client.py

@@ -587,6 +587,12 @@ def test_to_from_json(self):
 
     self.assertEqual(instance.__dict__, self.credentials.__dict__)
 
+  def test_from_json_token_expiry(self):
+    data = json.loads(self.credentials.to_json())
+    data['token_expiry'] = None
+    instance = OAuth2Credentials.from_json(json.dumps(data))
+    self.assertTrue(isinstance(instance, OAuth2Credentials))
+
   def test_no_unicode_in_request_params(self):
     access_token = u'foo'
     client_id = u'some_client_id'