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

[Andrew Barnert]

On Wednesday, August 13, 2014 12:45 PM, Guido van Rossum <guido at python.org> wrote:


>  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 just realized why this bothers me.

This function really, really ought to be taking an Iterable[String] (except that we don't have a String ABC). If you hadn't statically typed it, it would work just fine with, say, a text file—or, for that matter, a binary file. By restricting it to List[str], you've made it a lot less usable, for no visible benefit.

And, while this is less serious, I don't think it should be guaranteeing that the result is a Dict rather than just some kind of Mapping. If you want to change the implementation tomorrow to return some kind of proxy or a tree-based sorted mapping, you can't do so without breaking all the code that uses your function.

And if even Guido, in the motivating example for this feature, is needlessly restricting the usability and future flexibility of a function, I suspect it may be a much bigger problem in practice.


This example also shows exactly what's wrong with simple generics: if this function takes an Iterable[String], it doesn't just return a Mapping[String, int], it returns a Mapping of _the same String type_. If your annotations can't express that, any value that passes through this function loses type information. 

And not being able to tell whether the keys in word_count(f) are str or bytes *even if you know that f was a text file* seems like a pretty major loss.