Question

I’ve written a small snippet that computes the path length of a given node (e.g. its distance to the root node):

def node_depth(node, depth=0, colored_nodes=set()):
    """
    Return the length of the path in the parse tree from C{node}'s position
    up to the root node. Effectively tests if C{node} is inside a circle
    and, if so, returns -1.

    """
    if node.mother is None:
        return depth
    mother = node.mother
    if mother.id in colored_nodes:
        return -1
    colored_nodes.add(node.id)
    return node_depth(mother, depth + 1, colored_nodes)

Now there is a strange thing happening with that function (at least it is strange to me): Calling node_depth for the first time returns the right value. However, calling it a second time with the same node returns -1. The colored_nodes set is empty in the first call, but contains all node-IDs in the second call that have been added during first one:

print node_depth(node) # -->  9
# initially colored nodes --> set([])
print node_depth(node) # --> -1
# initially colored nodes --> set([1, 2, 3, 38, 39, 21, 22, 23, 24])

print node_depth(node, colored_nodes=set()) # --> 9
print node_depth(node, colored_nodes=set()) # --> 9

Am I missing some Python-specific thing here and this is really supposed to be that way?

Thanks in advance,

Jena

Answer 1

The “default value” for a function parameter in Python is instantiated at function declaration time, not every time the function is called. You rarely want to mutate the default value of a parameter, and so it’s often a good idea to use something immutable for the default value.

In your case you may want to do something like this:

def node_depth(node, depth=0, colored_nodes=None):
    ...
    if colored_nodes is None: colored_nodes = set()