Bug in python!? persistent value of an optional parameter in function!

[C Barr Leigh]

Help! Have I found a serious bug?
This seems like highly undesired behaviour to me. From the program
below, I get output:

call1: ['sdf']
call2: ['Set within test for call2']
call3: ['Set within test for call2']

instead of what I should get,

call1: ['sdf']
call2: ['Set within test for call2']
call3: ['Set within test for call3']

I'm using Python 2.4.4c1 (#2, Oct 11 2006, 21:51:02). The code is
below.


#!/usr/bin/python

def testPersistence(anarg,twooption=[]):
    #print anarg
    if not twooption:
        twooption.append('Set within test for '+anarg)
    print anarg +': '+str(twooption)

testPersistence('call1',twooption=['sdf']);
testPersistence('call2');
testPersistence('call3');