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Factory for Callback methods - Python TKinter

Answer 1

+1 A:

cb expects 'self' and 'event'. Maybe it only gets event from the bind?

SpliFF 2009-05-26 08:25:04

Yep, that was it! The 'self' was superfluous in the 'cb' method returned by 'cb_factory'. I will post the amended code to show the working example.Now if someone would kindly explain this to me, my brain is hurting!

Pev 2009-05-26 12:44:21

Answer 2

A:

Here is the amended code, taking into account SpliFF's answer. I find this much more aesthetically pleasing but it bugs me that I don't understand how it works. So, for extra credit, can anyone explain how this does work?

#!usr/bin/env python
"""
Python + Tk GUI interface to simulate a 8 Pio lines.
"""

from Tkinter import *
from pio_handler import *

class App( Frame ):
    """
    Main TK App class 
    """

    def __init__(self, parent):
        "Init"
        self.parent = parent
        self.bitfield = 0x00
        Frame.__init__(self, parent)

        self.messages = StringVar()
        self.messages.set("Initialised")

        Label( parent, bd=1, 
               relief=SUNKEN, 
               anchor=W, 
               textvariable=self.messages,
               text="Testing" ).pack(fill=X)

        self.bf_label = StringVar()
        self.bf_label.set("0 0 0 0 0 0 0 0")

        Label( parent, bd=1, 
               relief=SUNKEN, 
               anchor=W, 
               textvariable=self.bf_label,
               text="Testing" ).pack(fill=X)

        # This is the clever bit!
        # Use a factory to assign a callback function for keys 1 to 8
        for x in xrange(1,9):
            cb = self.cb_factory(x)
            self.parent.bind("<KeyPress-%d>" % x, cb) 
            self.parent.bind("<KeyRelease-%d>" % x, cb) 

    def display_bitfield(self):
        """
        Display the PIO lines (1 for on, 0 for off)
        """
        bin_lst = []
        for x in xrange(8):
            bit = 1 << x
            if bit & self.bitfield:
                bin_lst.append("1")
            else:
                bin_lst.append("0")
        bin_lst.reverse()
        bin_str = " ".join( bin_lst )
        self.bf_label.set( bin_str )

    def message( self, msg_txt ):
        "set"
        self.messages.set( msg_txt )

    def cb_factory(self,  numeric_key ):
        """
        Return a call back function for a specific keyboard numeric key (0-9)
        """
        def cb( event, key=numeric_key ):
            bit_val = 1<<numeric_key-1
            if int(event.type) == 2:
                self.bitfield |= bit_val
                self.message("Key %d Down" % key)
            else:
                self.bitfield &= (~bit_val & 0xFF)
                self.message("Key %d Up" % key)
            print hex(self.bitfield)
            self.display_bitfield()
        return cb

##########################################################################

if __name__ == "__main__":

    root = Tk()
    root.title("PIO Test")
    theApp = App( root )

    root.mainloop()

Pev 2009-05-26 12:50:31

Answer 3

A:

In answer to your followup question.

I'm not sure which part you don't understand but I'm guessing you don't quite have a handle on how event callbacks work? If so it's pretty easy. Tk runs in a loop looking for events (keypresses, mouseclicks, etc..). When you bind a callback/function to a event you're just telling it to call your function and pass an event object as it's argument. You can then query the event object for more details of what actually occurred. You are currently building seperate callback functions and binding each to 18 key events (down and release on keys 1-9). I think you can rewrite this to simply have cb as a method of your class because the event object will almost certainly contain the keycode as well.

class:
  def __init__(self):
    for x in xrange(8):
      self.parent.bind("<KeyPress-%d>" % x, self.keyaction)
      self.parent.bind("<KeyRelease-%d>" % x, self.keyaction)

  def keyaction(self, event):
    key = event.keycode # attribute may have another name, I haven't checked tk docs
    ... do stuff ...

Since we're now using self.keyaction as the callback it should get self as its first argument. It gets its keycode from the event object. Now neither value needs to be 'built' into the function when the function is created so the need to actually create different callbacks for each key is removed and the code easier to understand.

SpliFF 2009-05-26 14:03:10

It's key = event.charBut I was more interested in why the 'cb' method produced by the factory doesn't need 'self' as it's first parameter.

Pev 2009-05-27 07:26:28

because it inherited the meaning of self from the factory function, almost like a global variable. self isn't defined in cb but it is defined in the scope above (the object method)

SpliFF 2009-05-27 07:57:50

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