def append(self, obj):
        # Verify that the user is trying to add a Document object
        if not isinstance(obj, Document):
            raise TypeError("Only Document objects can be added to the \
document_list")
        # Check if the object is already in the list
        if [i.id for i in list(self.__iter__())] in obj.id:
            raise DuplicateObjectError("This object already exists in \
the document_list")
        # If it's all true, append it.
        super(DocumentSet, self).append(copy.copy(obj))
dataset/ETHPy150Open datadesk/python-documentcloud/documentcloud/__init__.py/DocumentSet.append/swapped operands of `in`