Tkinter callback arguments

Alf P. Steinbach alfps at start.no
Mon Nov 2 00:19:51 CET 2009


* MRAB:
> Lord Eldritch wrote:
>> Hi
>>
>> Maybe this is maybe something it has been answered somewhere but I 
>> haven't been able to make it work. I wanna pass one variable to a 
>> callback function and I've read the proper way is:
>>
>> Button(......, command=lambda: function(x))
>>
>> So with
>>
>> def function(a): print a
>>
>> I get the value of x. Ok. My problem now is that I generate the 
>> widgets in a loop and I use the variable to 'label' the widget:
>>
>> for x in range(0,3):  Button(......, command=lambda: function(x))
>>
>> so pressing each button should give me 0,1,2.
>>
>> But with the lambda, I always get the last index, because it gets 
>> actualized at each loop cycle. Is there any way to get that?
>>
> A lambda expression is just an unnamed function. At the point the
> function is /called/ 'x' is bound to 3, so that's why 'function' is
> always called with 3.
> 
> A function's default arguments are evaluated when the function is
> /defined/, so you can save the current value of 'x' creating the
> function (the lambda expression, in this case) with a default argument:
> 
>     for x in range(0,3):
>         Button(......, command=lambda arg=x: function(arg))
> 
> The following will also work, although you might find the "x=x" a bit
> surprising/confusing if you're not used to how Python works:
> 
>     for x in range(0,3):
>         Button(......, command=lambda x=x: function(x))

An alternative reusable alternative is to create a button-with-id class.

This is my very first Python class so I'm guessing that there are all sorts of 
issues, in particular naming conventions.

And the idea of creating a reusable solution for such a small issue may be 
un-pythonic?

But just as an example, in Python 3.x,


<code>
import tkinter
# I guess for Python 2.x do "import Tkinter as tkinter" but haven't tested.


class IdButton( tkinter.Button ):
     def __init__( self, owner_widget, id = None, command = None, **args ):
         tkinter.Button.__init__(
             self, owner_widget, args, command = self.__on_tk_command
             )
         self.__id = id
         self.__specified_command = command

     def __on_tk_command( self ):
         if self.__specified_command != None:
             self.__specified_command( self )
         else:
             self.on_clicked()

     def on_clicked( self ):
         pass
     def id( self ):
         return self.__id
     def id_string( self ):
         return str( self.id() );


def on_button_click( aButton ):
     print( "Button " + aButton.id_string() + " clicked!" )

window = tkinter.Tk()

n_buttons = 3
for x in range( 1, n_buttons + 1 ):
     IdButton(
         window, id = x, text = "Button " + str( x ), command = on_button_click
         ).pack()

window.mainloop()
</code>


Cheers,

- Alf

PS: Now I see that I've used camelCase once. Oh well...



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