I've never really seen arg1 depend on arg0 like that in Python.

However this comes up a lot for new engineers during programming interviews. It's very common to see recursive algos use empty lists as default/start-case args:

    def traverse(graph, seen_so_far=[]):  # Incorrect
        ....
        seen_so_far.append(node)

Subsequent calls to `traverse` will have state accumulated from earlier calls. The right solution is:

    def traverse(graph, seen_so_far=None):
        if not seen_so_far:
            seen_so_far = []
        .....

This isn't obvious if you're new to Python. This PEP makes this (arguably) clearer with:

    def traverse(graph, seen_so_far=>[]):