Basic Class/Instance Question

[Antoon Pardon]

On 2007-05-23, Bruno Desthuilliers <bruno.42.desthuilliers at wtf.websiteburo.oops.com> wrote:
> Siah a écrit :
>> Ready to go insane here. Class A, taking on a default value for a
>> variable. Instantiating two separate objects of A() gives me a shared
>> val list object. Just see the example bellow:
>> 
>> 
>> class A(object):
>>     def __init__(self, val=[]):
>>         self.val=val
>> 
>> obj1 = A()
>> obj2 = A()
>> 
>> print obj1 is obj2               # False - as expected
>> print obj1.val is obj2.val     # True! - Why... oh god WHY
> >
>> 
>> -----------
>> Using python 2.4. Is this a bug with this version of python? How can I
>> trust the rest of the universe is still in place? Could she still like
>> me? Many questions I have. Lets start with the python problem for now.
>
> This is a FAQ. Default arguments are only evaled once - when the def 
> statement is evaled (which is usually at import time). The solution is 
> simple: don't use mutable objects as default arguments:

An immutable object would have given the same behaviour in this case

class A(object):
    def __init__(self, val = ()):
        self.val=val

obj1 = A()
obj2 = A()

print obj1 is obj2               # False
print obj1.val is obj2.val       # True