Unexpected behavior when initializing class

[John Machin]

On Nov 28, 7:19 pm, "alfred.fa... at gmail.com" <alfred.fa... at gmail.com>
wrote:
> Hello everybody,
>
>     I've banged my ahead around for a while trying to figure out why
> multiple instances of a class share the same instance variable.  I've
> stripped down my code to the following, which reproduces my problem.
>
> class Test(object):
>     def __init__(self, v=[]):
>         self.values = v
>
>     def addValue(self, v):
>         self.values += [v]
>         return
>
> a = Test()
> a.addValue(1)
> print a.values   # Should print [1]
> b = Test()
> print b.values   # Should print empty list
> b.addValue(2)
> print a.values   # Should print [1]
>
> The output I get is:
>
> [1]
> [1]
> [1, 2]
>
> The output I am expecting is:
>
> [1]
> []
> [1]
>
> Another strange thing is that if I initialize with a different value,
> the new instance will not share the 'values' attribute with the other
> two:
>
> c = Test([9])
> print c.values    # Prints [9] as it should
> print a.values    # Still prints [1, 2]
>
> There is something I clearly don't understand here.  Can anybody
> explain?  Thanks!

http://www.python.org/doc/faq/general/#why-are-default-values-shared-between-objects

http://docs.python.org/tut/node6.html#SECTION006710000000000000000