[Python-ideas] Trial balloon: adding variable type declarations in support of PEP 484

[Guido van Rossum]

Regarding class variables: in my experience instance variables way
outnumber class variables, except when the latter are used as defaults
for instance variables.

See e.g. this bit of code from mypy (and many others, actually):
https://github.com/python/mypy/blob/master/mypy/nodes.py#L154. All of
these declare instance variables. Many of them have no need for a
default in the class, but must give one anyway or else there's nothing
to put the type comment on -- you can't write

    defs  # type: List[Statement]

(it would be a NameError) and you certainly don't want to write

    defs = []  # type: List[Statement]

(else the gods of shared mutable state will curse you) so you have to
make do with

    defs = None  # type: List[Statement]

But this would be totally reasonable as

    defs: List[Statement]

under my proposal.

On Mon, Aug 1, 2016 at 7:55 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> On Mon, Aug 01, 2016 at 02:31:16PM -0700, Guido van Rossum wrote:
>> PEP 484 doesn't change Python's syntax. Therefore it has no good
>> syntax to offer for declaring the type of variables, and instead you
>> have to write e.g.
>>
>> a = 0  # type: float
>> b = []  # type: List[int]
>> c = None  # type: Optional[str]
>>
>> I'd like to address this in the future, and I think the most elegant
>> syntax would be to let you write these as follows:
>>
>> a: float = 0
>> b: List[int] = []
>> c: Optional[str] = None
>
>
> Those examples look reasonable to me.
>
>
> [...]
>> Second, when these occur in a class body, they can define either class
>> variables or instance variables. Do we need to be able to specify
>> which?
>
> I would think so. Consider the case that you have Class.spam and
> Class().spam which may not be the same type. E.g. the class attribute
> (representing the default value used by all instances) might be a
> mandatory int, while the instance attribute might be Optional[int].
>
>
>> Third, there's an annoying thing with tuples/commas here. On the one
>> hand, in a function declaration, we may see (a: int = 0, b: str = '').
>> On the other hand, in an assignment, we may see
>>
>> a, b = 0, ''
>>
>> Suppose we wanted to add types to the latter. Would we write this as
>>
>> a, b: int, str = 0, ''
>>
>> or as
>>
>> a: int, b: str = 0, ''
>
> Require parens around the name:hint.
>
> (a:int), (b:str) = 0, ''
>
> Or just stick to a type hinting comment :-)
>
>
> What about this case?
>
> spam, eggs = [1, 2.0, 'foo'], (1, '')
> [a, b, c], [d, e] = spam, eggs
>
> That becomes:
>
> [(a:int), (b:float), (c:str)], [(x:int), (y:str)] = spam, eggs
>
> which is bearable, but just unpleasant enough to discourage people from
> doing it unless they really need to.
>
>
> --
> Steve
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-- 
--Guido van Rossum (python.org/~guido)