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functools.singledispatch interacts poorly with methods #76561

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ethanhs mannequin opened this issue Dec 19, 2017 · 7 comments
Closed

functools.singledispatch interacts poorly with methods #76561

ethanhs mannequin opened this issue Dec 19, 2017 · 7 comments
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3.8 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error

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@ethanhs
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ethanhs mannequin commented Dec 19, 2017

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
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    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
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    ethanhs mannequin commented Dec 19, 2017

    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.

    @ethanhs ethanhs mannequin added stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error labels Dec 19, 2017
    @ilevkivskyi
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    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.

    @ethanhs ethanhs mannequin added the 3.8 only security fixes label Mar 29, 2018
    @ncoghlan
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    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
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    ambv commented Apr 2, 2018

    +1

    @ambv
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    ambv commented May 26, 2018

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

    @ethanhs
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    ethanhs mannequin commented May 28, 2018

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

    @ethanhs ethanhs mannequin closed this as completed May 28, 2018
    @methane
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    methane commented Mar 27, 2019

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

    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
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