Consider contracts when evaluating Optional

While mypy considers assert to infer properties (e.g., is not None for Optional's), it would be great if it could also take contracts into account. Some properties can be easily inferred from contracts, in particular exclusivity.

Here is an example:

@icontract.ensure(lambda result: (result[0] is None) ^ (result[1] is None))
def some_func() -> Tuple[Optional[str], Optional[str]]:
    ...
    return something, errors

...

result, errors = some_func()
if errors is not None:
   ...

# We know here according to the contract that ``result`` can not be None.

The exclusivity is only but one example, and there are many more possible. I don't expect mypy to infer all the possible conclusions, but it would be already very nice to have at least some of the basic properties inferred.

There are a few contracts libraries around (e.g., icontract, for which I am the main author and maintainer, or deal) and they are gaining slowly some popularity.

I would be glad to help out on integration with icontract.

Non viene indicato alcun file, test o punto di ingresso. Inizia mappando il restringimento esistente di Optional in mypy a partire dalle istruzioni assert e l'integrazione proposta di icontract/deal; il lavoro sarebbe completato definendo e testando un insieme limitato di proprietà derivate dai contratti, come l'esclusività.