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

[Steven D'Aprano]

On Sun, Aug 17, 2014 at 12:13:24PM +0200, willvarfar at gmail.com wrote:
> When discussed on reddit programming ("proggit" as its called), there were
> plenty of people saying they really didn't like the mypy syntax:
> 
> http://www.reddit.com/r/programming/comments/2disob/proposal_for_python_type_annotations_from_guido/
> 
> (Declaration: I am the author of obiwan https://pypi.python.org/pypi/obiwan
> and I got a lot of upvotes on that proggit thread when I promoted the
> obiwan style instead)

My thoughts on the obiwan style?

Quicker, easier, more seductive. Easily they flow. But once you start 
down the dark path, forever will it dominate your destiny.

*wink*

Your example:

[mypy]
def word_count(input: List[str]) -> Dict[str, int]:

[obiwan]
def word_count(input: [str]) -> {str, int}:

Now consider that annotations are just expressions. Inside or outside of 
a function parameter definition, Dict[str, int] is exactly the same 
thing: it's an abstract type. I should be able to use it at runtime:

isinstance(x, word_count.__annotations__['return'])

But obiwan's style (like the old Jedi himself) twists the truth around. 
The return annotation is only a type from a certain perspective, but in 
reality it is an instance of a concrete class, not an abstract class.

It's not even a dict. {str, int} is a set, but obiwan uses it to 
represent a dict. (Although maybe that's a typo, and you meant to write 
{str: int}.)



-- 
Steven