Fix to_json of RelativeTime to support Isotime format.

common/lib/xmodule/xmodule/fields.py

@@ -196,7 +196,7 @@ class RelativeTime(Field):
         if isinstance(value, float):  # backward compatibility
             if value > 86400:
                 value = 86400
-            return str(datetime.timedelta(seconds=value))
+            return self._str(datetime.timedelta(seconds=value))
 
         if isinstance(value, datetime.timedelta):
             if value.total_seconds() > 86400:  # sanity check
@@ -204,18 +204,18 @@ class RelativeTime(Field):
                     "RelativeTime max value is 23:59:59=86400.0 seconds, "
                     "but {} seconds is passed".format(value.total_seconds())
                 )
-            return str(value)
+            return self._str(value)
 
         raise TypeError("RelativeTime: cannot convert {!r} to json".format(value))
 
-    def __str__(self):
+    def _str(self, value):
         """
-        Makes first H in str representation non-optional.
+        Makes first 'H' in str representation non-optional.
 
          str(timedelta) has [H]H:MM:SS format, which is not suitable
-         for front-end (and IsoTime standart), so we forse HH:MM:SS
+         for front-end (and IsoTime standart), so we forse HH:MM:SS format.
          """
-        _str = super(RelativeTime, slef).__str__(self)
-        if len(_str) == 7:
-            _str = '0' + _str
-        return _str
+        string = str(value)
+        if len(string) == 7:
+            string = '0' + string
+        return string

common/lib/xmodule/xmodule/tests/test_fields.py

@@ -145,7 +145,7 @@ class RelativeTimeTest(unittest.TestCase):
 
     def test_to_json(self):
         self.assertEqual(
-            "1:02:03",
+            "01:02:03",
             RelativeTimeTest.delta.to_json(datetime.timedelta(seconds=3723))
         )
         self.assertEqual(
@@ -153,7 +153,7 @@ class RelativeTimeTest(unittest.TestCase):
             RelativeTimeTest.delta.to_json(None)
         )
         self.assertEqual(
-            "0:01:40",
+            "00:01:40",
             RelativeTimeTest.delta.to_json(100.0)
         )
 
@@ -170,7 +170,7 @@ class RelativeTimeTest(unittest.TestCase):
 
     def test_str(self):
         self.assertEqual(
-            "1:02:03",
+            "01:02:03",
             RelativeTimeTest.delta.to_json(datetime.timedelta(seconds=3723))
         )
         self.assertEqual(