class A( object ):
    def __init__( self, someList = [] ):
        self.someList = someList

if __name__ == "__main__":
    for i in range( 10 ):
        a = A()
        a.someList.append( "abc" )
        print a.someList

Instead of each instance of A getting an empty list, it
is somehow the same list as the previous instance of A.
It will not occur with the following change:

class A( object ):
    def __init__( self ):
        self.someList = []

I'm using Windows XP, Python 2.4.1.
jason@griminventions.com

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This is intended behavior. Default values for function
arguments are only evaluated once, so it's not advisable to
use mutables there.

Use None as default and create your empty list within the
constructor if None is given, as in your second example.

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This is intended behavior. Default values for function
arguments are only evaluated once, so it's not advisable to
use mutables there.

Use None as default and create your empty list within the
constructor if None is given, as in your second example.

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This is not a bug.  Default values are evaluated when the
"def" statement is evaluated, at compile time.  See
http://www.python.org/doc/faq/general.html#why-are-default-values-shared-between-objects

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This is actually correct behavior. See the "Important
Warning" in this section of the tutorial:
http://docs.python.org/tut/node6.html#SECTION006710000000000000000