Using generator expressions to directly create tuples is a not so good idea performance-wise. Use it only if you have memory issues because it is not very performant. Using tuple([list comprenension]) is at most twice as fast.

# -*- coding: utf-8; mode: yaml; indent-tabs-mode: nil; -*-

# a subordinate duplicate key

%YAML 1.1
---

toplevel-key:
  subkey1: the value

  subkey2: 2nd value

  subkey1: not allowed