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

[Sven R. Kunze]

Another thought:

Can it be used for something else than type declarations like function 
annotations allow?


On 01.08.2016 23:31, 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
>
> (I've considered a 'var' keyword in the past, but there just are too
> many variables named 'var' in my code. :-)
>
> There are some corner cases to consider. First, to declare a
> variable's type without giving it an initial value, we can write this:
>
> a: float
>
> 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?
>
> 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, ''
>
> ??? Personally I think neither is acceptable, and we should just write it as
>
> a: int = 0
> b: str = ''
>
> but this is a slight step back from
>
> a, b = 0, ''   # type: (int, str)
>