[Python-Dev] PEP 484 update proposal: annotating decorated declarations

[Naomi Seyfer]

Stay tuned for the pep that allows callable to take keyword args. 

> On May 9, 2017, at 3:59 PM, Brett Cannon <brett at python.org> wrote:
> 
> The idea seems reasonable to me when viewing type hints as a form of documentation as it helps remind people how they are expected to call the final function.
> 
> One worry I do have, though, is Callable doesn't support keyword-only parameters, so declared_type won't work in all cases without Callable gaining such support (for those that don't know, Callable didn't start with that support as Callable has been meant for callback scenarios up to this point).
> 
>> On Tue, 9 May 2017 at 10:21 Guido van Rossum <guido at python.org> wrote:
>> There's a PR to the peps proposal here:
>> https://github.com/python/peps/pull/242
>> 
>> The full text of the current proposal is below. The motivation for this is that for complex decorators, even if the type checker can figure out what's going on (by taking the signature of the decorator into account), it's sometimes helpful to the human reader of the code to be reminded of the type after applying the decorators (or a stack thereof). Much discussion can be found in the PR. Note that we ended up having `Callable` in the type because there's no rule that says a decorator returns a function type (e.g. `property` doesn't).
>> 
>> This is a small thing but I'd like to run it by a larger audience than the core mypy devs who have commented so far. There was a brief discussion on python-ideas (my original, favorable reply by Nick, my response).
>> 
>> Credit for the proposal goes to Naomi Seyfer, with discussion by Ivan Levkivskyi and Jukka Lehtosalo.
>> 
>> If there's no further debate here I'll merge it into the PEP and an implementation will hopefully appear in the next version of the typing module (also hopefully to be included in CPython 3.6.2 and 3.5.4).
>> 
>> Here's the proposed text (wordsmithing suggestions in the PR please):
>> 
>> +Decorators
>> +----------
>> +
>> +Decorators can modify the types of the functions or classes they
>> +decorate. Use the ``decorated_type`` decorator to declare the type of
>> +the resulting item after all other decorators have been applied::
>> +
>> + from typing import ContextManager, Iterator, decorated_type
>> + from contextlib import contextmanager
>> +
>> + class DatabaseSession: ...
>> +
>> + @decorated_type(Callable[[str], ContextManager[DatabaseSession]])
>> + @contextmanager
>> + def session(url: str) -> Iterator[DatabaseSession]:
>> + s = DatabaseSession(url)
>> + try:
>> + yield s
>> + finally:
>> + s.close()
>> +
>> +The argument of ``decorated_type`` is a type annotation on the name
>> +being declared (``session``, in the example above). If you have
>> +multiple decorators, ``decorated_type`` must be topmost. The
>> +``decorated_type`` decorator is invalid on a function declaration that
>> +is also decorated with ``overload``, but you can annotate the
>> +implementation of the overload series with ``decorated_type``.
>> +
>> 
>> -- 
>> --Guido van Rossum (python.org/~guido)
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