Re: Problems with lambda functions in callbacks

Hello Dídac,

just to add an explanation, why your code doesn't work:

>from Tkinter import *
>
>root = Tk();
>
>def cb(x):
>    print x;
   
>for i in range(5):
>    b = Button(root,text=i,command=lambda:cb(i));

The part "lambda:cb(i)" is a function which gets
evaluated
(called) when a button is clicked. It is evaluated
in the scope where it is defined. In this scope (the global
scope)
the for-loop has finished running long ago.
Thus 'i' is always evaluated to '4' regardless which
button is pressed because this is the value of 'i'
after the end of the loop.

Stewart's code:
   b = Button(root,text=i,command=lambda i=i:cb(i));
This code works because it turns the loop variable 'i'
into a default value for the parameter 'i' of the lambda
function.
And default values are evaluated when the function is
defined,
i.e. while the loop is running.

>    b.pack();
>
>root.mainloop();

Hope this helps to demystify this behaviour,

Matthias Kievernagel
(mkiever/at/web/dot/de)
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Hi,

Thank you all for you responses. With Stewart's answer I
could get it 
working. Those were my first lines in python (that is why I
had so many 
semi-colons - I'm so used to C and C++).

And also thanks for the explanation. One really needs to
know how the 
programming language works when writing a program.

Cheers,

    Dídac

mkieverpytlink.de wrote:
> Hello Dídac,
>
> just to add an explanation, why your code doesn't work:
>
> >from Tkinter import *
>   
>> root = Tk();
>>
>> def cb(x):
>>    print x;
>>     
>    
>   
>> for i in range(5):
>>    b = Button(root,text=i,command=lambda:cb(i));
>>     
>
> The part "lambda:cb(i)" is a function which
gets evaluated
> (called) when a button is clicked. It is evaluated
> in the scope where it is defined. In this scope (the
global scope)
> the for-loop has finished running long ago.
> Thus 'i' is always evaluated to '4' regardless which
> button is pressed because this is the value of 'i'
> after the end of the loop.
>
> Stewart's code:
>    b = Button(root,text=i,command=lambda i=i:cb(i));
> This code works because it turns the loop variable 'i'
> into a default value for the parameter 'i' of the
lambda function.
> And default values are evaluated when the function is
defined,
> i.e. while the loop is running.
>
>   
>>    b.pack();
>>
>> root.mainloop();
>>     
>
> Hope this helps to demystify this behaviour,
>
> Matthias Kievernagel
> (mkiever/at/web/dot/de)
> _______________________________________________
> Tkinter-discuss mailing list
> Tkinter-discusspython.org
> http://mail.python.org/mailman/listinfo/tkinter-discuss
>
>
>   

-- 
------------------------------------------------------------
------------
*Dídac Busquets i Font*

Investigador Ramón y Cajal
Universitat de Girona
Dept. d'Electrònica, Informàtica i Automàtica
Institut d'Informàtica i Aplicacions
Edifici P-IV, Despatx 128
Campus Montilivi
17071 Girona

Tel. (+34) 972.41.81.79
Fax. (+34) 972.41.80.98
e-mail: busquetseia.udg.es
web: http://eia.udg.es/~busque
ts

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Abans d'imprimir aquest correu electrònic pensa si és
necessari fer-ho: 
el medi ambient és cosa de tots.

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