Unexpected behavior when initializing class

[alfred.fazio at gmail.com]

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!

Python 2.4.4 (#1, Oct 23 2006, 13:58:18)
[GCC 4.1.1 20061011 (Red Hat 4.1.1-30)] on linux2

Alfred J. Fazio,
afazio at smoothstone.com