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

[Chris Angelico]

On Thu, Aug 14, 2014 at 5:44 AM, Guido van Rossum <guido at python.org> wrote:
>   from typing import List, Dict
>
>   def word_count(input: List[str]) -> Dict[str, int]:
>       result = {}  #type: Dict[str, int]
>       for line in input:
>           for word in line.split():
>               result[word] = result.get(word, 0) + 1
>       return result

I strongly support the concept of standardized typing information.
There'll be endless bikeshedding on names, though - personally, I
don't like the idea of "from typing import ..." as there's already a
"types" module and I think it'd be confusing. (Also, "mypy" sounds
like someone's toy reimplementation of Python, which it does seem to
be :) but that's not really well named for "type checker using stdlib
annotations".) But I think the idea is excellent, and it deserves
stdlib support.

The cast notation sounds to me like it's what Pike calls a "soft cast"
- it doesn't actually *change* anything (contrast a C or C++ type
cast, where (float)42 is 42.0), it just says to the copmiler/type
checker "this thing is actually now this type". If the naming is clear
on this point, it leaves open the possibility of actual recursive
casting - where casting a List[str] to List[int] is equivalent to
[int(x) for x in lst]. Whether or not that's a feature worth adding
can be decided in the distant future :)

+1 on the broad proposal. +0.5 on defining the notation while leaving
the actual type checking to an external program.

ChrisA