Returns a content property.
        :param content_id (string): A string containing the id of the property content container.
        :param property_key (string): The key associated with the property requested.
        :param expand (string): OPTIONAL: A comma separated list of properties to expand on the content properties.
                                Default value: "version"
        :param callback: OPTIONAL: The callback to execute on the resulting data, before the method returns.
                         Default: None (no callback, raw data returned).
        :return: The JSON data returned from the content/{id}/property/{key} endpoint, or the results of the
                 callback. Will raise requests.HTTPError on bad input, potentially.

def get_content_property_by_key(self, content_id, property_key, expand=None, callback=None):
        """
        Returns a content property.
        :param content_id (string): A string containing the id of the property content container.
        :param property_key (string): The key associated with the property requested.
        :param expand (string): OPTIONAL: A comma separated list of properties to expand on the content properties.
                                Default value: "version"
        :param callback: OPTIONAL: The callback to execute on the resulting data, before the method returns.
                         Default: None (no callback, raw data returned).
        :return: The JSON data returned from the content/{id}/property/{key} endpoint, or the results of the
                 callback. Will raise requests.HTTPError on bad input, potentially.
        """
        params = {}
        if expand:
            params["expand"] = expand
        return self._service_get_request("rest/api/content/{id}/property/{key}".format(id=content_id, key=property_key),
                                         params=params, callback=callback)