Make EQ conditions work reliably in DynamoDB.

The AWS API represents a set object as a list of values. Internally
moto also represents a set as a list. This means that when we do value
comparisons, the order of the values can cause a set equality test to
fail.

moto/dynamodb2/models.py

@@ -66,6 +66,8 @@ class DynamoType(object):
                 return int(self.value)
             except ValueError:
                 return float(self.value)
+        elif self.is_set():
+            return set(self.value)
         else:
             return self.value
 
@@ -509,15 +511,12 @@ class Table(BaseModel):
                 elif 'Value' in val and DynamoType(val['Value']).value != current_attr[key].value:
                     raise ValueError("The conditional request failed")
                 elif 'ComparisonOperator' in val:
-                    comparison_func = get_comparison_func(
-                        val['ComparisonOperator'])
                     dynamo_types = [
                         DynamoType(ele) for ele in
                         val.get("AttributeValueList", [])
                     ]
-                    for t in dynamo_types:
+                    if not current_attr[key].compare(val['ComparisonOperator'], dynamo_types):
-                        if not comparison_func(current_attr[key].value, t.value):
+                        raise ValueError('The conditional request failed')
-                            raise ValueError('The conditional request failed')
         if range_value:
             self.items[hash_value][range_value] = item
         else:
@@ -946,15 +945,12 @@ class DynamoDBBackend(BaseBackend):
             elif 'Value' in val and DynamoType(val['Value']).value != item_attr[key].value:
                 raise ValueError("The conditional request failed")
             elif 'ComparisonOperator' in val:
-                comparison_func = get_comparison_func(
-                    val['ComparisonOperator'])
                 dynamo_types = [
                     DynamoType(ele) for ele in
                     val.get("AttributeValueList", [])
                 ]
-                for t in dynamo_types:
+                if not item_attr[key].compare(val['ComparisonOperator'], dynamo_types):
-                    if not comparison_func(item_attr[key].value, t.value):
+                    raise ValueError('The conditional request failed')
-                        raise ValueError('The conditional request failed')
 
         # Update does not fail on new items, so create one
         if item is None:

tests/test_dynamodb2/test_dynamodb_table_without_range_key.py

@@ -750,6 +750,47 @@ def test_boto3_update_item_conditions_pass_because_expect_exists_by_compare_to_n
     returned_item = table.get_item(Key={'username': 'johndoe'})
     assert dict(returned_item)['Item']['foo'].should.equal("baz")
 
+
+@mock_dynamodb2
+def test_boto3_update_settype_item_with_conditions():
+    class OrderedSet(set):
+        """A set with predictable iteration order"""
+        def __init__(self, values):
+            super(OrderedSet, self).__init__(values)
+            self.__ordered_values = values
+
+        def __iter__(self):
+            return iter(self.__ordered_values)
+
+    table = _create_user_table()
+    table.put_item(Item={'username': 'johndoe'})
+    table.update_item(
+        Key={'username': 'johndoe'},
+        UpdateExpression='SET foo=:new_value',
+        ExpressionAttributeValues={
+            ':new_value': OrderedSet(['hello', 'world']),
+        },
+    )
+
+    table.update_item(
+        Key={'username': 'johndoe'},
+        UpdateExpression='SET foo=:new_value',
+        ExpressionAttributeValues={
+            ':new_value': set(['baz']),
+        },
+        Expected={
+            'foo': {
+                'ComparisonOperator': 'EQ',
+                'AttributeValueList': [
+                    OrderedSet(['world', 'hello']),  # Opposite order to original
+                ],
+            }
+        },
+    )
+    returned_item = table.get_item(Key={'username': 'johndoe'})
+    assert dict(returned_item)['Item']['foo'].should.equal(set(['baz']))
+
+
 @mock_dynamodb2
 def test_boto3_put_item_conditions_pass():
     table = _create_user_table()