This behavior can be replicated with any class that has two special methods: __neg__ that returns -1 and __sub__ that accepts ints and returns 1-other.

For example if you make this class:

  class _:
       def __neg__(self):
           return -1
       def __sub__(self, other):
           return 1-other

  >>> --_()
  1
  >>> _()--_()
  2