Passing more arguments to this type of python function

Question

I figure this is pretty basic, but can't seem to figure out even how to ask google the right question. I am using this python websocket client to make some websocket connections. Let's just assume I'm using the code example similar to that page:

import websocket
import thread
import time

def on_message(ws, message):
    print(message)

def on_error(ws, error):
    print(error)

def on_close(ws):
    print("### closed ###")

def on_open(ws):
    def run(*args):
        ws.send("Hello")
        time.sleep(1)
        ws.close()
        print("thread terminating...")
    thread.start_new_thread(run, ())


if __name__ == "__main__":
    websocket.enableTrace(True)
    ws = websocket.WebSocketApp("ws://echo.websocket.org/",
                              on_message = on_message,
                              on_error = on_error,
                              on_close = on_close)
    ws.on_open = on_open
    ws.run_forever()

So what I am trying to do is add more arguments to the on_open function, something like this:

def on_open(ws, more_arg):
    def run(*args):
        ws.send("Hello %s" % more_arg)
        time.sleep(1)
        ws.close()
        print("thread terminating...")
    thread.start_new_thread(run, ())

But i can't figure out how to pass these arguments in, so I tried in the main thread:

ws.on_open = on_open("this new arg")

But i get the error:

TypeError: on_open() takes exactly 2 arguments (1 given)

How am I going to pass these new arguments to my on_open function?

Answer 1

Keep in mind that you need to assign a callback. You are instead calling a function and passing the return value to ws, which is incorrect.

You can use functools.partial to curry a function to a higher order one:

from functools import partial

func = partial(on_open, "this new arg")
ws.on_open = func

When func is invoked, it will invoke on_open with the first argument as "this new arg", followed by any other arguments passed to func. Look at the implementation of partial in the doclink for more details.

Answer 2

You can use a lambda to wrap the call:

ws.on_open = lambda *x: on_open("this new arg", *x)