def copy_object(self, container, obj, new_container, new_obj_name=None,
            content_type=None):
        """
        Copies the object to the new container, optionally giving it a new name.
        If you copy to the same container, you must supply a different name.

        Returns the etag of the newly-copied object.

        You can optionally change the content_type of the object by supplying
        that in the 'content_type' parameter.
        """
        nm = new_obj_name or utils.get_name(obj)
        uri = "/%s/%s" % (utils.get_name(new_container), nm)
        copy_from = "/%s/%s" % (utils.get_name(container), utils.get_name(obj))
        headers = {"X-Copy-From": copy_from,
                "Content-Length": "0"}
        if content_type:
            headers["Content-Type"] = content_type
        resp, resp_body = self.api.method_put(uri, headers=headers)
        return resp.headers.get("etag")