For your example I would do foo(Bar()) when I needed to run it without an existing object. Or make foo a method on the Bar class and do Bar().foo()

In general I just try my best to avoid immutable, or dynamic defaults, but sometimes they're necessary and I make that same if block at the top of the function. If I catch myself copy/pasting the same setup to multiple functions, then it might be time for a class or a decorator.