it has type checking.

pytyp has something similar - http://www.acooke.org/pytyp/pytyp.spec.record.html#module-py... - although that is just part of a wider framework for types. pytyp will also do things like use type annotations to map from json to python objects, for example, or check types on function args, or work with isinstance():

  >>> isinstance([1,2,None,4], Sequence(Option(int)))
  True

[although explicit types are not really pythonic (no argument there!), pytyp tries, within the whole "evil" concept of "adding types to python", to be as "true to the language" as possible. there's a long explanation at http://www.acooke.org/pytyp.pdf]