functools.singledispatch interacts poorly with methods

@ncoghlan

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']

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.

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.

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).

+1

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

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

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

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functools.singledispatch interacts poorly with methods #76561

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functools.singledispatch interacts poorly with methods #76561

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ethanhs commented on Dec 19, 2017

ilevkivskyi commented on Dec 22, 2017

ncoghlan commented on Mar 29, 2018

ambv commented on Apr 2, 2018

ambv commented on May 26, 2018

ethanhs commented on May 28, 2018

methane commented on Mar 27, 2019

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