Controlling assignation

[Bruno Desthuilliers]

Xavier Décoret a écrit :
(snip)
> What I wanted to do is something like this:
> 
> def change(x,v):
>     x = v
> 
> class A(object):
>     def __init__(self,v):
>         self.x = v
> 
> a = A(3)
> print a.x  # displays 3
> change(a.x,4)
> print a.x  # still displays 3
> 
> 
> It may seem weird, 

It does

> but I ensure there is a reason for doing this. 

I really wonder what it can be ???

>  In C++ 
> (the language I am mot familiar with), I could define f to take a 
> pointer to member function of a class, a pointer and a value and achieve 
> what I want. In python, I cannot that way becauswe when change(a.x,4) is 
> executed, a.x is "replaced" by ist value (returned by __getattribute__).
> 
> Finally, here is how I hold the situation:
> 
> 
> class Handle:
>     def __init__(self,v):
>         self.__v = v
>     def __call__(self):
>         x = self.__v
>         while callable(x): x=x()
>         return x
>     def set(self,v):
>         self.__v = v
> 
> class HandledProperty(object):
>     def __init__(self,name=""):
>         self.name = name
>     def __get__(self,o,t):
>         return o.__dict__[self]
>     def __set__(self,o,v):
>         o.__dict__[self] = Handle(v)
> 
> class A(object):
>     x = HandledProperty("x")
>     def __init__(self,v):
>         self.x = v
> 
> def change(x,v):
>     x.set(v)
> 
> 
> a = A(3)
> print a.x()  # displays 3 (notice the () in the call)
> change(a.x,4)
> print a.x()  # still displays 4

You could achieve the same effect with:

class A( object):
   def __init__(self, value):
     self.value = value

a = A(3)
a.value = 4
a.value
=> 4

And it's *much* more simple/readable/efficient.

Ever googled for "evolution of a programmer" ?-)