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

[Steven D'Aprano]

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