Does an equivalent of override exist for nested functions?

If I have this function, what should I do to replace the inner function with my own custom version?

def foo():
    def bar():
        # I want to change this
        pass

    # here starts a long list of functions I want to keep unchanged
    def baz():
        pass

Using classes this would be easily done overriding the method. Though, I can't figure out how to do that with nested functions. Changing foo to be a class (or anything else) is not an option because it comes from a given imported module I can't modify.