[Python-ideas] Proposal: Use mypy syntax for function annotations

[Oleg Broytman]

On Fri, Aug 15, 2014 at 04:52:45AM +1000, Steven D'Aprano <steve at pearwood.info> wrote:
> But with this proposal, Fred may write his function like this:
> 
> def foo(x:float)->float:
>     return (x+1)/2
> 
> and rely on mypy to check the types at compile time. Fred is happy: he 
> has static type checks, Python does it automatically for him (once he 
> has set up his build system to call mypy), and he is now convinced that 
> foo() is type-safe and an isinstance check at run-time would be a waste 
> of cycles.
> 
> I want to pass a Decimal to foo(). All I have to do is *not* install 
> mypy, or disable it, and lo and behold, like magic, the type checking 
> doesn't happen, and foo() operates by duck-typing just like in the glory 
> days of Python 1.5. Both Fred and I are now happy, and with the explicit 
> isinstance check removed, the only type checking that occurs when I run 
> Fred's library are the run-time duck-typing checks.

   Well, that's funny. Static type checking as a way to subvert type
checking! (-:

Oleg.
-- 
     Oleg Broytman            http://phdru.name/            phd at phdru.name
           Programmers don't die, they just GOSUB without RETURN.