Where is empty string passed?

The empty string "" is the default value for the protocol parameter. It's used when callers omit protocol, for example in Mount.url_path_for and Host.url_path_for:

In these cases the protocol is unknown at that point — it gets resolved later when a child route (like Route or WebSocketRoute) provides "http" or "websocket".

The empty string "" is the default value for the protocol parameter. It's used when callers omit protocol, for example in Mount.url_path_for and Host.url_path_for:

starlette/starlette/routing.py

Lines 420 to 427 in 348f86d

	def url_path_for(self, name: str, /, **path_params: Any) -> URLPath:
	if self.name is not None and name == self.name and "path" in path_params:
	# 'name' matches "<mount_name>".
	path_params["path"] = path_params["path"].lstrip("/")
	path, remaining_params = replace_params(self.path_format, self.param_convertors, path_params)
	if not remaining_params:
	return URLPath(path=path)
	elif self.name is None or name.startswith(self.name + ":"):

starlette/starlette/routing.py

Lines 486 to 492 in 348f86d

	def url_path_for(self, name: str, /, **path_params: Any) -> URLPath:
	if self.name is not None and name == self.name and "path" in path_params:
	# 'name' matches "<mount_name>".
	path = path_params.pop("path")
	host, remaining_params = replace_params(self.host_format, self.param_convertors, path_params)
	if not remaining_params:
	return URLPath(path=path, host=host)

In these cases the protocol is unknown at that point — it gets resolved later when a child route (like Route or WebSocketRoute) provides "http" or "websocket".

@Kludex

Thanks anyway. 🙏