FAQ

On 1 September 2015 at 23:03, Steven D'Aprano wrote:
PEP 484 says:

"No first-class syntax support for explicitly marking variables as being
of a specific type is added by this PEP. To help with type inference in
complex cases, a comment of the following format may be used: ..."

https://www.python.org/dev/peps/pep-0484/

I recall that in the discussions prior to the PEP, I got the strong
impression that Guido was open to the concept of annotating variables in
principle, but didn't think it was very important (for the most part,
the type checker should be able to infer the variable type), and he
didn't want to delay the PEP for the sake of agreement on a variable
declaration syntax when a simple comment will do the job.

So in principle, if we agree that type declarations for variables should
look like (let's say) `str s = some_function(arg)` then the syntax may
be added in the future, but it's a low priority.

The main case where it's potentially useful is when we want to
initialise a variable to None, but constrain permitted rebindings
(from a typechecker's perspective) to a particular type. When we
initialise a variable to an actual value, then type inference can
usually handle it.


Using the typing module as it exists today, I believe this should work
for that purpose (although I haven't actually tried it with mypy or
any other typechecker):


     from typing import TypeVar, Generic, Optional


     T = TypeVar("T")


     class Var(Generic[T]):
         def __new__(cls, value:Optional[T] = None) -> Optional[T]:
             return None


     i = Var[int]()


Unless I've misunderstood the likely outcome of type inference
completely, the value of i here will be None, but it's inferred type
would be Optional[int]. At runtime, you could still rebind "i" to
whatever you want, but a typechecker would complain if it was to
anything other than None or an integer.


Cheers,
Nick.


--
Nick Coghlan | ncoghlan at gmail.com | Brisbane, Australia

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