Default function arguments behaving badly

[jonny.longrigg at gmail.com]

Hi

I'm having some trouble with a function I've written in Python:

def myFunction(l1,l2,result=[]):
    index=0
    for i in l1:
        result.append([])
        if type(i)==list:
            myFunction(i,l2,result[index])
        else:
            for j in l2:
                result[index].append(i*j)
        index+=1
    return result

l1 and l2 are lists and the function multiplies every element of l1
with every element of l2. l1 is (possibly) multi-dimensional, and the
recursive bit is in there to cope with that. For example, if
l1=[[1,2],[3,4]] and l2=[5,6] the result is
[[[5,6],[10,12]],[[15,18],[20,40]]].

The problem is that it works if I run it once, but then on repeated
runs the value for 'result' doesn't seem to set itself to my default of
[], but instead uses the state it was in last time the function was
run.

I've had a problem like this in the past and ended up rewriting the
function as a class and using something like self.result, but I don't
really like this solution as the code becomes considerabley more
difficult to read (and I suspect less efficient). Also, I suppose I
could feed in an empty array every time but that doesn't provide a very
intuitive interface to the function.

Does anyone know what is going on here? Is there an easy solution?

Thanks for your help!