On 13.08.2014 21:44, Guido van Rossum wrote:
> Yesterday afternoon I had an inspiring conversation with Bob Ippolito
> (man of many trades, author of simplejson) and Jukka Lehtosalo (author
> of mypy: http://mypy-lang.org/). Bob gave a talk at EuroPython about
> what Python can learn from Haskell (and other languages); yesterday he
> gave the same talk at Dropbox. The talk is online
> (https://ep2014.europython.eu/en/schedule/sessions/121/) and in broad
> strokes comes down to three suggestions:
>
>   (a) Python should adopt mypy's syntax for function annotations
>   (b) Python's use of mutabe containers by default is wrong
>   (c) Python should adopt some kind of Abstract Data Types

I was at Bob's talk during EP14 and really liked the idea. A couple of
colleagues and other attendees also said it's a good and useful
proposal. I also like your proposal to standardize the type annotations
first without a full integration of mypy.

In general I'm +1 but I like to discuss two aspects:

1) I'm not keen with the naming of mypy's typing classes. The visual
distinction between e.g. dict() and Dict() is too small and IMHO
confusing for newcomers. How about an additional 'T' prefix to make
clear that the objects are referring to typing objects?

  from typing import TList, TDict

  def word_count(input: TList[str]) -> TDict[str, int]:
      ...

2) PEP 3107 only specifies arguments and return values but not
exceptions that can be raised by a function. Java has the "throws"
syntax to list possible exceptions:

 public void readFile() throws IOException {}

May I suggest that we also standardize a way to annotate the exceptions
that can be raised by a function? It's a very useful piece of
information and commonly requested information on the Python user
mailing list. It doesn't have to be a new syntax element, a decorator in
the typing module would suffice, too. For example:

  from typing import TList, TDict, raises

  @raises(RuntimeError, (ValueError, "is raised when input is empty"))
  def word_count(input: TList[str]) -> TDict[str, int]:
      ...

Regards,
Christian

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