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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def photos(self, username, page=1, per_page=10, order_by="latest"):
 """ Get a list of photos uploaded by a user. :param username [string]: The user’s username. Required. :param page [integer]: Page number to retrieve. (Optional; default: 1) :param per_page [integer]: Number of items per page. (Optional; default: 10) :param order_by [string]: How to sort the photos. Optional. (Valid values: latest, oldest, popular; default: latest) :return: [Array]: A single page of the Photo list. """

        url = "/users/{username}/photos".format(username=username)
        result = self._photos(url, username, page=page, per_page=per_page, order_by=order_by)
        return PhotoModel.parse_list(result)