Instantiation of 2nd object appends 1st?

[Steve Holden]

Carl Bray wrote:
> 
> In the following code A is instantiated with a list. A assigns the list to
> the attribute A_list.
> 
> My problem is that when I create a second instance of A the values in the
> second are those of the 1st AND 2nd instantiations rather than just the 2nd!
> 
> Why is this??
> 
> THE OUTPUT
> 
> [[1, 2, 3]]
> [[1, 2, 3], [1, 2, 3]]
> 
> THE CODE
> 
> class A:
> 
>   A_list = []
> 
This makes A_list an attribute of the class rather than each instance,
since the declaration is in the class scope rather than that of a method...

>   def __init__(self, the_list):
> 
>     # Append the list to A's list
>     self.A_list.append(the_list)
> 
Here you pick up the class attribute, even though you are qualifying the
instance with A_list.  And why appeand here if you don't want to build
an increasing list?  You could just assign a copy ... as in:

    def __init__(self, the_list):
        self.A_list = the_list[:]

That way, each instance gets its own attribute.  You should remove the
class attribute assignment, since this forces the name to be shared across
all instances.

>   def __repr__(self):
>     return repr(self.A_list)
> 
> for x in range(2):
> 
>   print A([1,2,3])
> 
> VERSION
> 
> Python 1.5.2 (#0, Apr 13 1999, 10:51:12) [MSC 32 bit (Intel)] on win32
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam

HTH

regards
 Steve
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