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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Callback functions arguments
Date: Mon, 27 Oct 2014 09:52:33 +0100
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ast wrote:

> 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 ?

Python doesn't "know" it has to pass an argument, it just does it. Change 
the callback to

def maj():
    print("no args")

and you'll get an error. If I were to guess

> echelle = Scale(Mafenetre, from_=-100, to=100, resolution=10, \
> orient=HORIZONTAL, length=300, width=20, label="Offset", \
> tickinterval=20, variable=Valeur, command=maj)

you probably are misled by the 'command=maj' part in the above line. This 
means that the function is passed and is different from command=maj() where 
the *result* of the function is passed. 

Here's a self-contained example that may clear things up for you:

>>> def call_them(one, two):
...     one(1)
...     two(2, 3)
... 
>>> def square(a):
...     print(a, "*", a, "=", a*a)
... 
>>> def product(a, b):
...     print(a, "*", b, "=", a*b)
... 
>>> call_them(square, product)
1 * 1 = 1
2 * 3 = 6
>>> call_them(product, product)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in call_them
TypeError: product() missing 1 required positional argument: 'b'

call_them() expects that one() takes 1 argument and two() takes 2 arguments. 
If the user passes a function that expects a different number of arguments a 
TypeError is raised.