> 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@python.org> wrote:

On Wed, Aug 13, 2014 at 9:06 PM, Jukka Lehtosalo <jlehtosalo@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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