Dynamically call methods in Python

It may be basic.

Why now? When I was communicating with WebSocket using tornado

Send callback method name with Javascript → Execute the method specified by tornado and send the callback function name to Javascript → Execute the callback function specified in Javascript

I wanted to call a method from a string.

First, connect to Javascript and WebSocket and send the calling method name and other parameters on the tornado side. The parameters are sent in JSON.

`python`


var ws = new WebSocket('ws://server/ws');
var p = {};
//callback method name
p['callback'] = 'wake_up';
//Other parameters
p['msg'] = 'Wake Up!!!!';
ws.send(JSON.stringify(p));
ws.onmessage = function(e) {
    var data = JSON.parse(e.data);
    console.log(data['message']);
};

The tornado side, getattr of on_message is the main part of this title.

`app.py`


from tornado import ioloop,web,websocket
import json
#callback class
from callback import Callback

class WSHandler(websocket.WebSocketHandler):
    #When the message comes
    def on_message(self, message):
        data = json.loads(message)
        #Instantiation of Callback class
        cb = Callback(data)
        #Method call
        result = getattr(cb, data['callback'])()
        
        #Value to return to Javascript
        return_value = {}
        return_value['message'] = result

        #Send message with WebSocket
        self.write_message(json.dumps(return_value))

handlers = [
    (r'/ws', WSHandler),
]

settings = dict(
    debug = False,
)

app = web.Application(handlers, **settings)
app.listen(sys.argv[1])
ioloop.IOLoop.instance().start()

The method to be called.

`callback.py`


class Callback(data):
    def __init__(self, data):
        self.data = data

    def wake_up(self):
        return self.data['msg']

You should now see'Wake Up !!!!'on your browser console.

The usage of getattr is as follows. Details are left to the official documentation.

`python`


cb = Callback(data)
result = cb.wake_up()

#The above is the same as the code below
result2 = getattr(cb, 'wake_up')()

#By the way, if you do as follows, the return value will not be returned and the object will be assigned.
result3 = getattr(cb, 'wake_up')

Did you do it like result3 above? ?? ?? But when you think about it, it's right. .. ..

2013-03-15 postscript Fixed a subtle mistake. Excuse me.