Allows specifying timedeltas in expressions involving TTLAttribute (#665)

For example, the following will be supported:
```
class MyModel(Model):
  ...
  ttl = TTLAttribute()

model = MyModel()
model.update(actions=[
  MyModel.ttl.set(timedelta(days=42)),
])
```

Author: ikonst

pynamodb/attributes.py

@@ -503,30 +503,35 @@ class TTLAttribute(Attribute):
     """
     attr_type = NUMBER
 
-    def __set__(self, instance, value):
+    def _normalize(self, value):
         """
-        Converts assigned values to a UTC datetime
+        Converts value to a UTC datetime
         """
+        if value is None:
+            return
         if isinstance(value, timedelta):
             value = int(time.time() + value.total_seconds())
         elif isinstance(value, datetime):
             if value.tzinfo is None:
                 raise ValueError("datetime must be timezone-aware")
             value = calendar.timegm(value.utctimetuple())
-        elif value is not None:
+        else:
             raise ValueError("TTLAttribute value must be a timedelta or datetime")
-        attr_name = instance._dynamo_to_python_attrs.get(self.attr_name, self.attr_name)
-        if value is not None:
-            value = datetime.utcfromtimestamp(value).replace(tzinfo=tzutc())
-        instance.attribute_values[attr_name] = value
+        return datetime.utcfromtimestamp(value).replace(tzinfo=tzutc())
+
+    def __set__(self, instance, value):
+        """
+        Converts assigned values to a UTC datetime
+        """
+        super(TTLAttribute, self).__set__(instance, self._normalize(value))
 
     def serialize(self, value):
         """
         Serializes a datetime as a timestamp (Unix time).
         """
         if value is None:
             return None
-        return json.dumps(calendar.timegm(value.utctimetuple()))
+        return json.dumps(calendar.timegm(self._normalize(value).utctimetuple()))
 
     def deserialize(self, value):
         """

tests/test_attributes.py

@@ -517,6 +517,11 @@ def test_ttl_attribute_wrong_type(self):
             model = AttributeTestModel()
             model.ttl_attr = 'wrong type'
 
+    @patch('time.time')
+    def test_serialize_timedelta(self, mock_time):
+        mock_time.side_effect = [1559692800]  # 2019-06-05 00:00:00 UTC
+        assert TTLAttribute().serialize(timedelta(seconds=60)) == str(1559692800 + 60)
+
     def test_serialize_none(self):
         model = AttributeTestModel()
         model.ttl_attr = None

tests/test_model.py

@@ -190,6 +190,7 @@ class Meta:
     views = NumberAttribute(null=True)
     is_active = BooleanAttribute(null=True)
     signature = UnicodeAttribute(null=True)
+    ttl = TTLAttribute(null=True)
 
 
 class CustomAttrIndex(LocalSecondaryIndex):
@@ -806,10 +807,12 @@ def test_delete_doesnt_do_validation_on_null_attributes(self):
                 car = CarModel('foo')
                 batch.delete(car)
 
-    def test_update(self):
+    @patch('time.time')
+    def test_update(self, mock_time):
         """
         Model.update
         """
+        mock_time.side_effect = [1559692800]  # 2019-06-05 00:00:00 UTC
         self.init_table_meta(SimpleUserModel, SIMPLE_MODEL_TABLE_DATA)
         item = SimpleUserModel('foo', is_active=True, email='[email protected]', signature='foo')
 
@@ -839,7 +842,8 @@ def test_update(self):
                 SimpleUserModel.is_active.set(None),
                 SimpleUserModel.signature.set(None),
                 SimpleUserModel.custom_aliases.set(['bob']),
-                SimpleUserModel.numbers.delete(0, 1)
+                SimpleUserModel.numbers.delete(0, 1),
+                SimpleUserModel.ttl.set(timedelta(seconds=60)),
             ])
 
             args = req.call_args[0][1]
@@ -851,14 +855,15 @@ def test_update(self):
                         'S': 'foo'
                     }
                 },
-                'UpdateExpression': 'SET #0 = :0, #1 = :1, #2 = :2, #3 = :3 REMOVE #4 DELETE #5 :4',
+                'UpdateExpression': 'SET #0 = :0, #1 = :1, #2 = :2, #3 = :3, #4 = :4 REMOVE #5 DELETE #6 :5',
                 'ExpressionAttributeNames': {
                     '#0': 'email',
                     '#1': 'is_active',
                     '#2': 'signature',
                     '#3': 'aliases',
-                    '#4': 'views',
-                    '#5': 'numbers'
+                    '#4': 'ttl',
+                    '#5': 'views',
+                    '#6': 'numbers'
                 },
                 'ExpressionAttributeValues': {
                     ':0': {
@@ -874,6 +879,9 @@ def test_update(self):
                         'SS': ['bob']
                     },
                     ':4': {
+                        'N': str(1559692800 + 60)
+                    },
+                    ':5': {
                         'NS': ['0', '1']
                     }
                 },