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

[Stefan Behnel]

Guido van Rossum schrieb am 14.08.2014 um 07:24:
> On Wed, Aug 13, 2014 at 9:06 PM, Jukka Lehtosalo 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.)

That's how Cython's "fused types" (generics) work, at least. They go by
name: same name of the type, same type. Otherwise, use alias names, which
make the types independent from each other.

http://docs.cython.org/src/userguide/fusedtypes.html

While it's a matter of definition what way to go here (same type or not),
practice has shown that it's clearly the right decision to make identical
types the default.

Stefan