singledispatch support for type annotations

[ambv]

BPO	32227
Nosy	@gvanrossum, @rhettinger, @ambv, @1st1, @ilevkivskyi, @csabella
PRs	bpo-32227: functools.singledispatch supports registering via type annotations #4733 bpo-27984: explictly check type of singledispatch.dispatch() argument #6114

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/ambv'
closed_at = <Date 2018-03-14.23:34:12.078>
created_at = <Date 2017-12-05.21:24:12.510>
labels = ['3.7', 'type-feature', 'library']
title = 'singledispatch support for type annotations'
updated_at = <Date 2018-03-14.23:34:12.077>
user = 'https://github.com/ambv'

bugs.python.org fields:

activity = <Date 2018-03-14.23:34:12.077>
actor = 'lukasz.langa'
assignee = 'lukasz.langa'
closed = True
closed_date = <Date 2018-03-14.23:34:12.078>
closer = 'lukasz.langa'
components = ['Library (Lib)']
creation = <Date 2017-12-05.21:24:12.510>
creator = 'lukasz.langa'
dependencies = []
files = []
hgrepos = []
issue_num = 32227
keywords = ['patch']
message_count = 8.0
messages = ['307686', '307690', '307691', '307692', '307693', '308075', '311565', '311599']
nosy_count = 6.0
nosy_names = ['gvanrossum', 'rhettinger', 'lukasz.langa', 'yselivanov', 'levkivskyi', 'cheryl.sabella']
pr_nums = ['4733', '6114']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32227'
versions = ['Python 3.7']

[ambv]

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:

@singledispatch
def generic(arg): ...

@generic.register
def _(arg: str): ...

@generic.register
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.

[ambv]

Guido, I know that you have a lot on your plate right now. Adding you because of PEP-443 and PEP-484.

[ilevkivskyi]

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.

[ilevkivskyi]

Oops something wrong happened.

[ambv]

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.

[ambv]

New changeset e569753 by Łukasz Langa in branch 'master':
bpo-32227: functools.singledispatch supports registering via type annotations (bpo-4733)
e569753

[csabella]

Hello,

I believe this could be closed as resolved? Do you think it should be added to the What's New? page for 3.7?

[ilevkivskyi]

Do you think it should be added to the What's New? page for 3.7?

I leave this up to Łukasz.