[Tutor] class instance with identity crisis

[Jeff Shannon]

Barnaby Scott wrote:

> class Damson:
>     def __str__(self):
>         return 'damson'
>     def dry(self):
>         self = Prune()
> 
> class Prune:
>     def __str__(self):
>         return 'prune'
> 
> weapon = Damson()
> weapon.dry()
> print weapon
> 
> [...]
> 
> but something in me suggests it should produce
> 
> prune
> 
> After all, 'self' refers to the instance 'weapon'.

Ah, but 'self' is a method-local name.  You're re-binding that name, 
not affecting the object (previously) pointed to by the name.  And of 
course, when the method returns, those local bindings go away.

> Obviously one could reassign weapon to a Prune outside the class definition,
> but I was hoping to write something in which, given certain circustances
> arising, the instance would change itself into something else.

As others have mentioned, you can change the type of the object by 
assigning to self.__class__, but this can get a bit hairy.  Your 
class/instance knows nothing about what names it may be bound to, so 
it can't rebind those names to a different object (i.e. class Damson, 
and any instances you create, know nothing of the name 'weapon').

I tend to favor the explicit approach of having weapon.dry() *return* 
a different object,  and then using 'weapon = weapon.dry()'.  But if 
you really need to have objects that change their type as a 
side-effect of a method call, you can use a proxy or adapter -- in 
essence, you have a class that can *contain* either a Damson or a 
Prune, and which forwards method calls / attribute accesses to its 
contained object.

Jeff Shannon
Technician/Programmer
Credit International