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

[Haoyi Li]

> I heartily agree. But just for the type theorists amongst us, if I really
wanted to write the most general type, how would I express that the AnyStr
in the return type matches the one in the argument? (I think pytypedecl
would use something like T <= AnyStr.)

To borrow Scala syntax, it would look something like

def word_count[AnyStr <: String](input: Iterable[AnyStr]): Dict[AnyStr, Int]

Where *word_count* is a generic function on the type *AnyStr*, which is not
just *any* type but a type bound by the restriction it is a subclass of
*String*. Thus you can force that the *AnyStr* going in and the *AnyStr*
going out are the same one.

I'm not sure if mypy allows for type-bounds, but that's the way you achieve
what you want if it does, or will, in future =P


On Wed, Aug 13, 2014 at 10:24 PM, Guido van Rossum <guido at python.org> wrote:

> On Wed, Aug 13, 2014 at 9:06 PM, Jukka Lehtosalo <jlehtosalo at gmail.com>
> wrote:
>>
>>
>> You could use AnyStr to make the example work with bytes as well:
>>
>>   def word_count(input: Iterable[AnyStr]) -> Dict[AnyStr, int]:
>>       result = {}  #type: Dict[AnyStr, int]
>>
>>       for line in input:
>>           for word in line.split():
>>               result[word] = result.get(word, 0) + 1
>>       return result
>>
>> Again, if this is just a simple utility function that you use once or
>> twice, I see no reason to spend a lot of effort in coming up with the most
>> general signature. Types are an abstraction and they can't express
>> everything precisely -- there will always be a lot of cases where you can't
>> express the most general type. However, I think that relatively simple
>> types work well enough most of the time, and give the most bang for the
>> buck.
>>
>
> I heartily agree. But just for the type theorists amongst us, if I really
> wanted to write the most general type, how would I express that the AnyStr
> in the return type matches the one in the argument? (I think pytypedecl
> would use something like T <= AnyStr.)
>
> --
> --Guido van Rossum (python.org/~guido)
>
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