python string vs list odd behaviour
<p>If I try the following snippets of code on a binary tree, and try to print the arr and string later, arr gives me the correct result but string is empty. Any thoughts? Is it something to do with lists being passed by reference and strings passed by value? </p>

<pre><code>def post_order(root, arr = []):
    if(root is not None):
        post_order(root.left, arr)
        post_order(root.right, arr)
        arr.append(root.value)

def post_order1(root, string = ''):
    if(root is not None):
        post_order1(root.left, string)
        post_order1(root.right, string)
        string += str(root.value)

# assume I've made my binary tree
arr, string = [], ''
post_order(root, arr)
post_order1(root, string)
print arr, string
# arr holds the correct post-order sequence
# string is empty
</code></pre>