Callback functions arguments

Jean-Michel Pichavant jeanmichel at sequans.com
Mon Oct 27 11:19:17 CET 2014


----- Original Message -----
> From: "ast" <nomail at invalid.com>
> To: python-list at python.org
> Sent: Monday, 27 October, 2014 9:16:26 AM
> Subject: Callback functions arguments
> 
> Hi
> 
> In this web site at example n°5
> http://fsincere.free.fr/isn/python/cours_python_tkinter.php
> 
> A program is using the "Scale" widget from tkinter module.
> Here is a piece of code:
> 
> Valeur = StringVar()
> 
> echelle = Scale(Mafenetre, from_=-100, to=100, resolution=10, \
> orient=HORIZONTAL, length=300, width=20, label="Offset", \
> tickinterval=20, variable=Valeur, command=maj)
> 
> The "maj" callback function is:
> 
> def maj(nouvelleValeur):
>     print(nouvelleValeur)
> 
> When the user move the scale with the mouse, the new position
> is supposed to be printed on the python shell.
> 
> The "maj" function has an argument "nouvelleValeur" but no
> argument is passed through the Scale widget.
> 
> So how the hell Python knows that it has to pass parameter
> "Valeur" to the "maj" function ?
> 
> thx

The Scale object is performing the call, hence it will be the Scale object that will call your maj function with a "nouvelleValeur" parameter.

When you write command=maj, you pass the function, but you don't call it. That's the purpose of a callback. You provide a function and it get called by the object you've been giving the function to. The Scale object should be documented and should provide with the callback signature.

See http://effbot.org/zone/tkinter-callbacks.htm

JM


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