functools.singledispatch interacts poorly with methods

[ethanhs]

BPO	32380
Nosy	@ncoghlan, @methane, @ambv, @ilevkivskyi, @ethanhs
PRs	bpo-32380 Fix singledispatch interaction with methods #4987 bpo-32380: Create functools.singledispatchmethod #6306 bpo-32380: add "versionadded: 3.8" to singledispatchmethod #12580

^{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 = None
closed_at = <Date 2018-05-28.22:47:25.724>
created_at = <Date 2017-12-19.22:23:08.394>
labels = ['3.8', 'type-bug', 'library']
title = 'functools.singledispatch interacts poorly with methods'
updated_at = <Date 2019-03-27.09:15:21.725>
user = 'https://github.com/ethanhs'

bugs.python.org fields:

activity = <Date 2019-03-27.09:15:21.725>
actor = 'methane'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-28.22:47:25.724>
closer = 'ethan smith'
components = ['Library (Lib)']
creation = <Date 2017-12-19.22:23:08.394>
creator = 'ethan smith'
dependencies = []
files = []
hgrepos = []
issue_num = 32380
keywords = ['patch']
message_count = 7.0
messages = ['308687', '308944', '314644', '314817', '317765', '317941', '338945']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'methane', 'lukasz.langa', 'levkivskyi', 'ethan smith']
pr_nums = ['4987', '6306', '12580']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32380'
versions = ['Python 3.8']

[ethanhs]

Consider the following:

from functools import singledispatch

class Dispatch:
    @singledispatch
    def foo(self, a):
        return a

    @foo.register(int)
    def _(self, a):
        return "int"

    @foo.register(str)
    def _(self, a):
        return "str"

cls = Dispatch()
cls.foo(3)  # 3
cls.foo('hm')  # 'hm'

I find this quite unintuitive. Essentially, since singledispatch dispatches based solely on a functions first argument, it is useless on methods unless one wraps it and modifies how it uses the internal wrapper function. I believe this should be relatively easy to fix with adding a check of inspect.ismethod and then modifying the number of the checked argument where appropriate.

I'm happy to write a patch if this change is seen as a good idea.

[ilevkivskyi]

I have also noticed this problem and I like the idea. It appeared some time ago on python-ideas, but no one has written a patch.

[ncoghlan]

Added Łukasz to the nosy list, as I'd like his +1 before accepting this change (I do still think it's a good idea, for the same reasons we added partialmethod).

[ambv]

+1

[ambv]

New changeset c651275 by Łukasz Langa (Ethan Smith) in branch 'master':
bpo-32380: Create functools.singledispatchmethod (bpo-6306)
c651275

[ethanhs]

This was fixed, so I think it can be closed.

[methane]

New changeset bc284f0 by Inada Naoki in branch 'master':
bpo-32380: add "versionadded: 3.8" to singledispatchmethod (GH-12580)
bc284f0