Title: singledispatch support for type annotations
Type: enhancement Stage:
Components: Library (Lib) Versions: Python 3.7
Status: open Resolution:
Dependencies: Superseder:
Assigned To: lukasz.langa Nosy List: gvanrossum, levkivskyi, lukasz.langa, rhettinger, yselivanov
Priority: normal Keywords: patch

Created on 2017-12-05 21:24 by lukasz.langa, last changed 2017-12-11 21:56 by lukasz.langa.

Pull Requests
URL Status Linked Edit
PR 4733 merged lukasz.langa, 2017-12-05 21:26
Messages (6)
msg307686 - (view) Author: Łukasz Langa (lukasz.langa) * (Python committer) Date: 2017-12-05 21:24
With the patch attached to this issue, @singledispatch gains support for passing the type in @register via annotations.

This looks more natural and enables more thorough type checking without repeating yourself:

def generic(arg): ...

def _(arg: str): ...

def _(arg: int): ...

The previous API is still available for backwards compatibility, as well as stacking, and use with classes (sic, I was surprised to learn it's used that way, too).

The patch should be uncontroversial, maybe except for the fact that it's importing the `typing` module if annotations are used. This is necessary because of forward references, usage of None as a type, and so on. More importantly, with PEP 563 it's mandatory.
msg307690 - (view) Author: Łukasz Langa (lukasz.langa) * (Python committer) Date: 2017-12-05 21:40
Guido, I know that you have a lot on your plate right now. Adding you because of PEP 443 and PEP 484.
msg307691 - (view) Author: Ivan Levkivskyi (levkivskyi) * (Python committer) Date: 2017-12-05 21:42
Idea looks interesting (like a basic runtime @overload). My expectation is that some changes are necessary in mypy for this to work properly.

Another (minor) problem is that this creates a circular dependency functools <-> typing.
msg307692 - (view) Author: Ivan Levkivskyi (levkivskyi) * (Python committer) Date: 2017-12-05 21:43
Oops something wrong happened.
msg307693 - (view) Author: Łukasz Langa (lukasz.langa) * (Python committer) Date: 2017-12-05 21:58
> this creates a circular dependency functools <-> typing

Well, it doesn't since I explicitly import typing inside singledispatch. By the time this import happens, functools is fully imported. This would only be a problem if functools itself tried to use singledispatch with annotations. Not impossible but super unlikely.
msg308075 - (view) Author: Łukasz Langa (lukasz.langa) * (Python committer) Date: 2017-12-11 21:56
New changeset e56975351bc2c574c728f738e88effba7116279f by Łukasz Langa in branch 'master':
bpo-32227: functools.singledispatch supports registering via type annotations (#4733)
Date User Action Args
2017-12-11 21:56:33lukasz.langasetmessages: + msg308075
2017-12-05 21:58:51lukasz.langasetmessages: + msg307693
2017-12-05 21:43:57levkivskyisetnosy: + gvanrossum

messages: + msg307692
stage: patch review ->
2017-12-05 21:42:42levkivskyisetnosy: + levkivskyi, - gvanrossum
messages: + msg307691
stage: patch review
2017-12-05 21:40:04lukasz.langasetnosy: + gvanrossum

messages: + msg307690
stage: patch review -> (no value)
2017-12-05 21:26:44lukasz.langasetkeywords: + patch
stage: patch review
pull_requests: + pull_request4636
2017-12-05 21:24:12lukasz.langacreate