Function args

[Peter Otten]

Jean-Michel Caricand wrote:

> Imaginons que je veuille écrire une fonction qui permute deux variables,
> j'écris :
> 
> def permuter(a,b):
>   c = b
>   b = a
>   a = c
> 
> Si tout est objet, donc passé par référence a et b devraient être
> réellement modifier ? Pourtant ce n'est pas le cas.

Assignments inside a function only affect local bindings, i. e what a and b
"point" to in the local context. Rebinding in the calling context must be
made explicit:

>>> a = 1
>>> b = 2
>>> a, b = swap(a, b)
>>> a
2
>>> b
1

Of course you wouldn't write a function for that particular example:

>>> a = 1
>>> b = 2
>>> a, b = b, a
>>> a
2
>>> b
1
>>>

Does the above mean you can never change an object from inside a function?
No, only bindings cannot be changed and because integers are "immutable",
i. e. cannot be changed once they are created, what is viewed as changing
them is in fact rebinding. Now a simple example with a mutable class:

>>> class A:
...     def __init__(self, name, value):
...             self.name = name
...             self.value = value
...     def __repr__(self):
...             return "A(name=%r, value=%r)" % (self.name, self.value)
...
>>> def swapAttr(a, b):
...     a.value, b.value = b.value, a.value
...
>>> a = A("A", 1)
>>> b = A("B", 2)
>>> a, b
(A(name='A', value=1), A(name='B', value=2))
>>> swapAttr(a, b)
>>> a, b
(A(name='A', value=2), A(name='B', value=1))

While objects stay the same, you can do anything with the attributes. As
Michel Claveau already pointed out, the same is true for list and dict
items. 

Peter