converting base class instance to derived class instance

Sridhar R sridharinfinity at yahoo.com
Tue Jan 20 13:13:37 CET 2004


Consider the code below,

class Base(object):
  pass

class Derived(object):

  def __new__(cls, *args, **kwds):
    # some_factory returns an instance of Base
    # and I have to derive from this instance!
    inst = some_factory() # this returns instance of Base
    return inst  # Oops! this isn't an instance of Derived

  def __init__(self):
    # This won't be called as __new__ returns Base's instance
    pass

  The constrait is there is some factory function that creates an
instance of Base class.  So I _can't_ call "Base.__init__(self)" in
Derived.__init__ func, as their will be _two_ instances (one created
by Derived.__init__ and other created by the factory function)

  Simply I want to get a derived class object, but instead of allowing
_automatic_ creation of Base instance, I want to use _existing_ Base's
instance, then use this as a Derived's instance.

  Will metaclass solve this problem?



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