Ok

  if a is None:
    raise Exception()
  if a.b is None:
    raise Exception()
  if a.b.c is None:
    raise Exception()

One of the advantages of Python/Typescript style type hinting is that it avoids the mega-nested `if`s that you often have to resort to in Rust. In Rust the above would not work:

  if a.is_none() {
    bail!();
  }
  if a.b. // nope! a is still Option<>.

It may be possible to fix that when enum variants are distinct types but as it stands you often have to do lots of indenting.