Copy construction of class instance object

[Gonçalo Rodrigues]

On Tue, 27 May 2003 20:30:00 GMT, "Bror Johansson" <bjohan at telia.com>
wrote:

>
>"Peter Hansen" <peter at engcorp.com> wrote in message
>news:3ED3C1DE.8EC82FC2 at engcorp.com...
>> Bror Johansson wrote:
>> >
>> > Is there a good/recommended way to emulate the copy constructor
>> > classinstance creation (a la C++) in Python?
>>
>> "import copy" plus an appropriate line or two?  See
>> http://www.python.org/doc/2.0/lib/module-copy.html
>
>My try:
>
>import copy
>
>class A(object):
>    def __init__(self, ainst):
>        self.val = 5
>
>class B(A):
>    def __init__(self, ainst):
>        self = copy.deepcopy(ainst)
>

Besides what G. Holl said in another thread, notice that you are not
calling A's __init__ so B instances never get the val attribute.

>>> class A(object):
... 	def __init__(self):
... 		self.val = 5
... 		
>>> class B(A):
... 	def __init__(self):
... 		pass
... 	
>>> b = B()
>>> b.val
Traceback (most recent call last):
  File "<interactive input>", line 1, in ?
AttributeError: 'B' object has no attribute 'val'
>>> 


In Python super class initializers/constructors/whatever are never
called automatically.

>a = A()
>b= B(a)
>
>print b.val
>
>=> AttributeError: 'B' object has no attribute 'val'
>

HTH, with my best regards,
G. Rodrigues