[Python-ideas] PEP 484 (Type Hints) -- second draft

[Ryan Gonzalez]

On Mon, Mar 23, 2015 at 2:55 PM, Guido van Rossum <guido at python.org> wrote:

> On Sun, Mar 22, 2015 at 11:02 PM, David Foster <davidfstr at gmail.com>
> wrote:
>
>> * It's a bit vague how local variables should be typed in the presence of
>>   multiple assignments. Consider:
>>
>>   ```
>>   if x > 0:
>>       root = sqrt(x)  # type: int
>>   else:
>>       root = None
>>   ```
>>
>>   It seems that the type of `root` should be inferred as `Union[int,
>> None]`
>>   (AKA `Optional[int]`), but there's no discussion about this type of
>>   inference in the PEP.
>>
>>   Cases like this are the only way that the proposed blank `Undefined`
>>   value makes sense to me, using an example like:
>>
>>   ```
>>   root = Undefined    # type: Optional[int]
>>   if x > 0:
>>       root = sqrt(x)  # type: int
>>   else:
>>       root = None
>>   ```
>>
>
> Right. Another use case for Undefined is class variables (serving as
> instance variable initialization/declaration).
>
> If you feel strongly that this needs to be mentioned in the PEP, can you
> send a PR to github.com/ambv/typehinting ?
>
>
>> * +1 for only allowing .pyi as the extension for stub files (and not also
>> .py).
>>   Makes it very clear that they should only be interpreted as stubs.
>>
>
> Perhaps. Though there are also downsides.
>
>
>> * -1 that `Optional[T1]` is unnecessary. It's conceptually more clean and
>>   directly corresponds conceptually to types like `int?` and `?int` from
>>   other languages.
>>
>
> Indeed, Optional[t] is much clearer to the human reader than Union[t,
> None].
>
>
>> * +1 for the argument order cast(x, T). This is consistent with (x: T)
>> elsewhere.
>>
>
> I disagree on this. It goes against the argument order of casts in other
> languages, e.g. C, C++, Java, and even in Python -- you write int(x), not
> x(int).
>

I brought this up somewhere before, but I still think this would be great
for casts:

cast[T](x)

Like C++'s named casts (static_cast<T>(x), dynamic_cast<T>(x), etc.). It
gives the notion that T is a type, not some random variable, and that cast
is NOT a normal function call.


>
> --
> --Guido van Rossum (python.org/~guido)
>
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-- 
Ryan
[ERROR]: Your autotools build scripts are 200 lines longer than your
program. Something’s wrong.
http://kirbyfan64.github.io/
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