I wonder, why that kind of ambiguity or complexity even comes to your mind at all. Just because python is weird?

def foo(self, arg=expression):

could, and should work as if it was written like this (pseudocode)

def foo(self, arg?): if is_not_given(arg): arg=expression

if "expression" is a literal or a constructor, it'd be called right there and produce new object, if "expression" is a reference to an object in outer scope, it'd be still the same object.

it's a simple code transformation, very, very predictable behavior, and most languages with closures and default values for arguments do it this way. Except python.