Ugh. You're just ignoring my entire post except the one part where you (wrongly) think you can correct me?

The code I posted is showing how you can explicitly get the mutable behavior if you want it when the default expressions are evaluated at call time.

That is to say, if you evaluate default expressions at call time, you can get either behavior by being explicit with minimal loss in performance:

    def foo(xs = []):
        xs.append(1)
        print(xs) # always prints [1]

    DEFAULT = []
    def bar(xs = DEFAULT):
        xs.append(1)
        print(xs) # prints [1], [1,1], [1,1,1], etc.

To reiterate, the above code is what would happen if default expressions were evaluated at function call time.

You're clearly not as knowledgeable as you think you are on this and you're just cherry picking things you don't understand to feel smart.