[Python-Dev] Is static typing still optional?

[Steve Holden]

On Thu, Dec 21, 2017 at 7:55 PM, Terry Reedy <tjreedy at udel.edu> wrote:

> On 12/21/2017 9:23 AM, Eric V. Smith wrote:
>
>>
>>
>> On 12/21/17 6:25 AM, Sven R. Kunze wrote:
>>
>>> On 21.12.2017 11:22, Terry Reedy wrote:
>>>
>>>>
>>>> @dataclass
>>>>>> class C:
>>>>>>      a: int # integer field with no default
>>>>>>      b: float = 0.0 # float field with a default
>>>>>>
>>>>>> And the types will be recognized by type checkers such as mypy.
>>>>>>
>>>>>> And I think the non-typed examples should go first in the docs.
>>>>>>
>>>>>
>>>>
>>> I still don't understand why "I don't care" can be defined by "leaving
>>> out"
>>>
>>> @dataclass
>>> class C:
>>>      b = 0.0 # float field with a default
>>>
>>
>> Because you can't know the order that x and y are defined in this example:
>>
>> class C:
>>      x: int
>>      y = 0
>>
>> 'x' is not in C.__dict__, and 'y' is not in C.__annotations__.
>>
>
>
​Solely because, annotations being optional, the interpreter is not allowed
to infer from its presence  that an annotated name should be ​allocated an
entry in __dict__, and clearly the value associated with it would be
problematical.

I think the understanding problem with this feature arises from two
> factors: using annotations to define possibly un-initialized slots is
> non-obvious; a new use of annotations for something other than static
> typing is a bit of a reversal of the recent pronouncement 'annotations
> should only be used for static typing'.  Therefore, getting the permanent
> doc 'right' is important.
>

​Indeed. So annotations are optional, except where they aren't?​


> The following naively plausible alternative does not work and cannot
> sensibly be made to work because the bare 'x' in the class scope, as
> opposed to a similar error within a method, causes NameError before the
> class is created.
>
> @dataclass
> class C:
>     x
>     y = 0
>
> ​Quite. Could this be handled the same way not-yet initilialised slots
are? (Pardon my ignornace).
​


> I think the doc should explicitly say that uninitialized fields require
> annotation with something (anything, not necessarily a type) simply to
> avoid NameError during class creation.  It may not be obvious to some
> readers why x:'anything' does not also raise NameError, but that was a
> different PEP, and the dataclass doc could here link to wherever
> name:annotation in bodies is explained.
>
>
​This contortion is why I feel a better solution would be desirable. Alas I
do not have one to hand.

regards
 Steve​
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