[Python-Dev] Please reject or postpone PEP 526

[Ivan Levkivskyi]

On 4 September 2016 at 13:30, Mark Shannon <mark at hotpy.org> wrote:

> It would be a real shame if PEP 526 mandates against checkers doing as
> good as job as possible. Forcing all uses of a variable to have the same
> type is a major and, IMO crippling, limitation.
>
> E.g.
> def foo(x:Optional[int])->int:
>     if x is None:
>        return -1
>     return x + 1
>
> If the type of the *variable* 'x' is Optional[int] then 'return x + 1'
> doesn't type check. If the type of the *parameter* 'x' is Optional[int]
> then a checker can readily verify the above code.
>

Mark,

First, in addition to the quote from my previous e-mail, I would like to
show another quote from PEP 526
"This PEP does not require type checkers to change their type checking
rules. It merely provides a more readable syntax to replace type comments"

Second, almost exactly your example has been added to PEP 484:

class Reason(Enum):
    timeout = 1
    error = 2

def process(response: Union[str, Reason] = '') -> str:
    if response is Reason.timeout:
        return 'TIMEOUT'
    elif response is Reason.error:
        return 'ERROR'
    else:
        # response can be only str, all other possible values exhausted
        return 'PROCESSED: ' + response

I think mypy either already supports this or will support very soon (and
the same for Optional)

--
Ivan
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