[Tutor] Hi everybody stuck on some error need help please thank you!!

[Steven D'Aprano]

On Sat, 24 Apr 2010 01:07:11 pm Marco Rompré wrote:

> Here's my code:
[...]
> class Modele:
>     """
>     La definition d'un modele avec les magasins.
>     """
>     def __init__(self, nom_fichier, magasins =[]):
>         self.nom_fichier = nom_fichier
>         self.magasins = magasins
[...]
> if __name__ == '__main__':
>     modele = Modele()
>     nom_fichier = "magasinmodele.txt"
>     modele.charger(nom_fichier)
>     if modele.vide():
>         modele.initialiser(nom_fichier)
>     modele.afficher()
>
> And here's my error :
>
> Traceback (most recent call last):
>   File "F:\School\University\Session 4\Programmation
> SIO\magasingolfmodele.py", line 187, in <module>
>     modele = Modele()
> TypeError: __init__() takes at least 2 arguments (1 given)


You define Modele to require a nom_fichier argument, but then you try to 
call it with no nom_fuchier.


Also, I see that you do this:

def __init__(self, nom_fichier, magasins =[]):

You probably shouldn't do this -- it doesn't do what you probably think 
it does.

You probably think that what happens is that if you call 
Modele(nom_fichier), the magasins attribute will be set to an empty 
list. But that's not what happens.

Default values in Python are defined when the method is created, not 
when it is run. Watch this example:

>>> class Test:
...     def __init__(self, x=[]):
...             self.x = x
...
>>> a = Test()
>>> a.x
[]
>>> b = Test()
>>> b.x
[]
>>>
>>> a.x.append("Surprise!")
>>> b.x
['Surprise!']


How does this happen? Because every instance shares the same default 
list. When you append to it, all the other instances see the same 
change.

You don't notice this with default values that are strings or numbers, 
because you can't modify them, only replace them:

>>> x = y = 2  # Make two variables that point to the same value.
>>> x is y  # Make sure they are identical, not just equal.
True
>>> x = 3  # Make x point to something else.
>>> x is y  # And no longer identical.
False
>>>
>>> x = y = []  # Make two variables that point to the same thing.
>>> x is y
True
>>> x.append('something')  # Modify that list in place.
>>> x is y  # Still identical.
True
>>> y
['something']


If this is the behaviour you want, then you don't need to do anything. 
Otherwise you need to move the creation of the empty list inside the 
method:

    def __init__(self, nom_fichier, magasins=None):
        if magasins is None:
            magasins = []
        self.nom_fichier = nom_fichier
        self.magasins = magasins



-- 
Steven D'Aprano