Using an existing instance as parent

Bruno Desthuilliers bruno.42.desthuilliers at websiteburo.invalid
Mon Sep 1 17:08:36 CEST 2008

André a écrit :
> Hi, I was trying to find a way to set, upon __init__() the parent of a
> class to an existing instance.

??? Sorry but I just can't make any sense of this.

> Here is a minimal example of what I'm
> trying to do:
> class A(object):
>   def __init__(self, x):
>     self.x = x
> class B(A):
>   def __init__(self, *args):
>     if not isinstance(args[0], A):
>       super(B, self).__init__(args[0])
>     else:
>       self = args[0]

Rebinding an argument only affect the current namespace.

>     self.y = args[1]
> b = B(4, 6)
> print 'b:', b.x, b.y, type(b)
> a = A(7)
> c = B(a, 3) # Means: please set c parent's using instance "a"
> print 'c:', c.x, c.y, type(c)
> This does not work as can be tested. The reason I'm in search for a
> solution in this area is that in our project, "A" is not copy-able (it
> is written using a boost.python binding to a C++ object that does not
> allow copying) - so I can't simply call, inside "B's __init__()", a
> copy constructor for A.

Ok, I think I get the point... When an A instance is passed as first 
arg, you'd like to use this instance's state to call super(B).__init__ ?

If so, here's a possible solution (Q&D, to be corrected according to 
real use case, etc):

import copy

class B(A):
   def __init__(self, *args):
     if isinstance(args[0], A):
         x = copy.copy(args[0].x) # if it's a mutable and you want a copy
         x = args[0]

    super(B, self).__init__(x)
    self.y = args[1]

Be sure to carefully read copy's doc.

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