[Tutor] Learning about callbaks

[Michael Bernhard Arp Sørensen]

Greetings, my master.

I'm writing a game based on curses.

I have my own screen object and several child objects to handle sub windows
with e.g. menues, board/map/views and log outputs. All user input is done
with screen.getch and later sent to the dynamic menu for selecting menu
points.

My imidiate problem is when I select "Quit" from the menu, I need to send
the string back to the caller/parent class for evaluation.

Later I will need to design all the menues and the related methods, other
sub windows in different threads for individual updates. But first I need a
working UI.

Off topic: I must say that I'm amazed by this tutor thing. To really have my
"own" tutor in this new programming language I'm learning, is kinda blowing
my mind. I hope I can repay the python community some day when I'm smart
enough. :-)

Thanks in advance

/karneevor

On Dec 29, 2007 6:39 PM, Alan Gauld <alan.gauld at btinternet.com> wrote:

>
> "Michael Bernhard Arp Sørensen" <michaelarpsorensen at stevnstrup.dk>
> wrote
>
> > I want to learn about callbacks because we use it at work in our
> > software.
>
> Can you be more specific about what you want to know. Callbacks are
> used in many different ways from event handling methods in a GUI
> to network programming to simulating synchronous protocols over
> an asynchronous connection.
>
> > I there a short "hello world"-like version of a callback example?
>
> See almost any GUI tutorial.
> The recent thread "Closing GUI program" had the following example
> from Michael Goldwasser
>
> #-------------------------------------------
> from Tkinter import Tk,Label
>
> def onClose():
>    root.destroy()   # stops the main loop and interpreter
>
> root = Tk()
> root.protocol("WM_DELETE_WINDOW", onClose)  # handle event when window
> is closed by user
> z = Label(root, text="Hello World!")
> z.grid()
> root.mainloop()
> #-------------------------------------------
>
>
> In this example the onClose() event handler is a callback function.
>
> The folowing pseusdo code shows how the principle can be used for
> asynchronous network programming:
>
> waiting = {}     # list of objects awaiting responses
> id = 0
>
> def sendToServer(msg, callback)
>       msgSent = prepareMessage(msg)
>       id = server.send(msgSent)
>       waiting[id] = (msg, callback)
>
> def func1()
>     msg = prepareData()
>     sendToServer(msg, func1_cont)
>
> def func1_cont(original, result)
>     x,y,z = result.getValues()
>     processData(x,y,z,original.p,original.q)
>
> while server.listen()
>      msg = server.recv()
>      id = msg.getID()
>      oldMessage = waiting[id][0]
>      callback =  waiting[id][1]
>      callback(oldmessage, msg)
>      del(waiting[id])
>
> In this example we can think of the main application calling func.
> func1 needs to send a message to a server and process the response
> but the server has an asynchronous protocol so we split the function
> into func1 and func1_cont at the point of calling the server. Then
> when the server send us the response we pull the stored state out
> of the dictionary and combine it with the server data to complete
> the func1 processing via the func1_cont callback.
>
> In practice we'd probably store the date/time with the transaction
> data so that we can check for timeouts etc in a separate thread...
>
> The important thing with all callbacks is that you match up the
> data expected by the callback with the data actually available
> at the point of calling it. In this case we take the architectural
> decision to pass callback functions the old and new data structures.
> We could alternatively have passed the transaction id and let the
> callback retrieve (and delete) the data from the waiting list.
>
> I hope that all makes sense.
>
> --
> Alan Gauld
> Author of the Learn to Program web site
> http://www.freenetpages.co.uk/hp/alan.gauld
>
>
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
>



-- 
Med venlig hilsen/Kind regards

Michael B. Arp Sørensen
Programmør / BOFH
I am /root and if you see me laughing you better have a backup.
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