[Python-Dev] PEP 484 update proposal: annotating decorated declarations (original) (raw)

Guido van Rossum guido at python.org
Tue May 9 13:19:39 EDT 2017

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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 <https://mail.python.org/pipermail/python-ideas/2017-April/045548.html>, favorable reply <https://mail.python.org/pipermail/python-ideas/2017-May/045550.html> by Nick, my response <https://mail.python.org/pipermail/python-ideas/2017-May/045557.html>).

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.

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