Add exception reporting to ISODatetime

Summary:
We see exceptions from this validator in prod, but we only record the error message
we return, which deprives us of the details of the real exception.
This adds exception logging when we see failures so that we can see what is really happening.
The added passing test case is a value that appears to fail in the wild.

Test Plan: A new test.

Reviewers: naphat

Reviewed By: naphat

Subscribers: changesbot, anupc

Differential Revision: https://tails.corp.dropbox.com/D225616

changes/api/validators/datetime.py

@@ -1,11 +1,15 @@
-from __future__ import absolute_import, print_function
+from __future__ import absolute_import
 
 from datetime import datetime
 
+import logging
+
 
 class ISODatetime(object):
     def __call__(self, value):
+        # type: (str) -> datetime
         try:
             return datetime.strptime(value, '%Y-%m-%dT%H:%M:%S.%fZ')
         except Exception:
+            logging.exception("Failed to parse datetime: %s", value)
             raise ValueError('Datetime was not parseable. Expected ISO 8601 with timezone: YYYY-MM-DDTHH:MM:SS.mmmmmmZ')

tests/changes/api/validators/test_datetime.py

@@ -0,0 +1,20 @@
+from datetime import datetime
+
+import pytest
+from changes.api.validators.datetime import ISODatetime
+
+
+def test_isodatetime_valid():
+    validator = ISODatetime()
+    assert validator('2016-09-01T19:46:18.9Z') == datetime(year=2016,
+                                                           month=9,
+                                                           day=1,
+                                                           hour=19,
+                                                           minute=46,
+                                                           second=18,
+                                                           microsecond=900000)
+
+
+def test_isodatetime_invalid():
+    with pytest.raises(ValueError):
+        ISODatetime()('invalid')