Instance variable reference

[navyad]

class Attachment(object):

    def to_dict(self):
        return {
            'type': self.attachment_type,
            'payload': self.payload
        }

The above class does not have the attachment_type and payload attributes, I guess you have kept this to be used by child classes, which make sense.

Although creating the instance of Attachment is not the idea, But instance can be created. And in that case
to_dict() will break, since the attachment_type and payload attributes are not there.

There are two solution of the problem:
Solution 1: allows instantiation of Attachment and add attributes to the class. Will not recommend it since the idea is not to allow the instantiation of this class.

Solution 2: Does not allow instance of class while maintaining the inheritance of the attributes.

[geeknam]

The proper way is to use python's abc, felt like it wasn't worth the effort.

[navyad]

@geeknam thanks for suggesting, abc module provides elegant way to handle such scenario.

related change is here