copy construtor question

[Jan Kaliszewski]

28-08-2009 o 20:38:30 xiaosong xia <xiaosongxia at yahoo.com> wrote:

> I am trying to define a class with copy constructor as following:
>  
> class test:
>      def __init__(self, s=None):
>           self=s
>  
> x=[1,2]
>  
> y=test(x)
>  
> print y.__dict__
>  
> it gives
> {}
>  
> The above code doesn't work.

And cannot, as Chris has already written. But it is possible to
customize construction of objects of your class -- using __new__():
[see: http://docs.python.org/reference/datamodel.html#object.__new__ ]

     import copy
     # in Python 2.x you must explicitly inherit from 'object' type
     class Test(object):
         "Not tested"
         def __new__(cls, s, way=None):
             if s is None:
                 # a new object of 'Test' class
                 return super(currentclass, cls).__new__(cls, *args)
             else:
                 if way == 'alias':
                     return s
                 elif way == 'shallow':
                     return copy.copy(s)
                 elif way == 'deep':
                     return copy.deepcopy(s)
                 else:
                     raise ValueError("'way' must be 'alias', "
                                      "'shallow' or 'deepcopy'")

...but in such cases as copying existing objects it is usualy better
(though less romantic :-)) to use an ordinary function (e.g. simply
copy.copy() or copy.deepcopy(), as Gabriel has pointed).

Regards,
*j

-- 
Jan Kaliszewski (zuo) <zuo at chopin.edu.pl>