[Tutor] odd behavior within __init__

[Rich Krauter]

Orri Ganel wrote:
> Hello all,
> 
> As part of a project i'm doing (mostly for the fun of it), I have a
> class which creates a sort of wrapper around any object to make it
> suitable for use in a custom container.  However, if the class
> receives an already wrapped object, I want it to just return the
> object (same id and everything as the original). Now, the following
> seems to work in the __init__ method (due to output), but then it
> disappears as soon as the __init__ method is left:
> 
> class Node:
> ...
> def __init__(self, cargo=None, prev=None, next=None, nod=False):
>                """x.__init__(...) initializes x; see
> x.__class__.__doc__ for signature"""
>                if not isinstance(cargo, Node) or nod:
>                    self.cargo = cargo
>                    self.prev = prev
>                    self.next = next
>                else:
>                    self = cargo
>                    print id(self), id(cargo)
>                    print self.cargo
> 
> 
>>>>a = Node(1)
>>>>b = Node(a)
> 
> 12932600 12932600
> 1
> 
>>>>id(b)
> 
> 12960632
> 
> Any ideas on why this happens, or suggestions as to how to implement
> the behavior I'm looking for (in which b and a would refer to the same
> object, have the same id, etc.), would be greatly appreciated.
> 
> Thanks in advance,
> Orri
> 


Orri,

Maybe you could use a factory. It would allow you to simplify your Node 
class, and encapsulate the instantiation behavior you want outside the 
class.


class Node(object):
     def __init__(self,cargo=None,prev=None,next=None):
         self.cargo = cargo
         self.prev = prev
         self.next = next

def factory(cargo=None,prev=None,next=None):
     if isinstance(cargo,Node):
         return cargo
     else:
         return Node(cargo,prev,next)


n1 = factory(1)
print id(n1)
n2 = factory(n1)
print id(n2)


Good luck,

Rich