Kenny Li wrote:
> Kent:
>
> I forgot to mention that no coping is allowed. Your two options
> essentially are doing copying of the b.
Not really. I make new references to the attributes of b.
It sounds like you want
c = C(b)
to actually convert b to be an instance of C, instead of creating a new
object. Is that right? You can do that it C.__new__ but I wonder why you
want to?
Here is some code that might do what you want:
''' Construction of a derived class from a base class returns the base class
converted to an instance of the derived class '''
class B(object):
''' the baseClass '''
def __init__(self, arg1, arg2):
self.a1=arg1
self.a2=arg2
def __repr__(self):
return 'B(%r, %r)' % (self.a1, self.a2)
class C(B):
''' C is subClass of B '''
def __new__(cls, arg1, arg2=None):
if isinstance(arg1, B):
# If given a B instance, convert it to a C and return it
arg1.__class__ = C
return arg1
return B.__new__(cls, arg1, arg2)
def __init__(self, arg1, arg2=None):
if not isinstance(arg1, B):
B.__init__(self, arg1, arg2)
self.extra="blah blah blah"
def __repr__(self):
return 'C(%r, %r, %r)' % (self.a1, self.a2, self.extra)
b=B(1, 2)
print b
c=C(2, 3)
print c
c=C(b)
print c, (c is b)
print b # b is now a C - it's the same object as c
Kent
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