one example is `namedtuple`'s `._replace`: >>> x = Foo(a=1, b=2, c=3) >>> x._replace(a=1000, c=3000) Foo(a=1000, b=2, c=3000) in this case, you really n

one example is `namedtuple`'s `._replace`: >>> x = Foo(a=1, b=2, c=3) >>> x._replace(a=1000, c=3000) Foo(a=1000, b=2, c=3000) in this case, you really need to know if the user passed a replacement or not.