PEP 526 - var annotations and the spirit of python

Steven D'Aprano steve+comp.lang.python at pearwood.info
Thu Jul 5 09:07:12 EDT 2018


On Thu, 05 Jul 2018 13:54:28 +0200, Antoon Pardon wrote:

> On 05-07-18 11:59, Steven D'Aprano wrote:
>> On Thu, 05 Jul 2018 17:34:55 +1200, Gregory Ewing wrote:
>>
>>
>>>> Indeed, that's often the best way, except for the redundant type
>>>> hint, which makes you That Guy:
>>>>
>>>>     x: int = 0  # set x to the int 0
>>> But you've shown in an earlier example that such a hint is *not*
>>> always redundant
>> But it is redundant in *that* example. Your hint is not giving any more
>> information that what the reader, or type checker, can already infer.
> 
> Yes it gives more information. Without the hint you can only interfere
> that it is an int at that point.

That would be utterly pointless:

 x = 3                  # infer x must be an int, but only at this point
 y = x + "Hello world"  # who knows what x is now? maybe it's okay


A type checker that did that would be a waste of time.


> IIUC, with the hint you can interfere
> it is supposed to be an int during its life time, not just at that
> point.

No.

    Mypy considers the initial assignment as the definition
    of a variable. If you do not explicitly specify the type
    of the variable, mypy infers the type based on the
    static type of the value expression

http://mypy.readthedocs.io/en/latest/type_inference_and_annotations.html

This is not an innovation of Mypy. It's how type inference is supposed to 
work. If a particular type checker doesn't do that, it is doing it wrong.



-- 
Steven D'Aprano
"Ever since I learned about confirmation bias, I've been seeing
it everywhere." -- Jon Ronson



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