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functools.singledispatchfunction has confusing error message if no positional arguments are passed in #85294

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mgrandi mannequin opened this issue Jun 26, 2020 · 2 comments
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functools.singledispatchfunction has confusing error message if no positional arguments are passed in #85294

mgrandi mannequin opened this issue Jun 26, 2020 · 2 comments
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3.13 new features, bugs and security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error

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@mgrandi
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Mannequin

mgrandi mannequin commented Jun 26, 2020

BPO 41122
Nosy @rhettinger, @ambv, @mgrandi, @ammaraskar, @mental32, @AlexWaygood
PRs
  • gh-85294: Handle missing arguments to @singledispatchmethod gracefully #21471
  • bpo-41122: Check for required positional argument in singledispatchmethod invocation #23212
    		•

    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 = None
created_at = <Date 2020-06-26.03:29:31.194>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'functools.singledispatchfunction has confusing error message if no positional arguments are passed in'
updated_at = <Date 2021-11-05.16:40:42.304>
user = 'https://github.com/mgrandi'

    bugs.python.org fields:

    activity = <Date 2021-11-05.16:40:42.304>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-06-26.03:29:31.194>
creator = 'markgrandi'
dependencies = []
files = []
hgrepos = []
issue_num = 41122
keywords = ['patch']
message_count = 1.0
messages = ['372406']
nosy_count = 6.0
nosy_names = ['rhettinger', 'lukasz.langa', 'markgrandi', 'ammar2', 'mental', 'AlexWaygood']
pr_nums = ['21471', '23212']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue41122'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']
    @mgrandi
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    Mannequin Author

    mgrandi mannequin commented Jun 26, 2020

    this is with python 3.8:

    PS C:\Users\mark> py -3 --version --version
Python 3.8.2 (tags/v3.8.2:7b3ab59, Feb 25 2020, 23:03:10) [MSC v.1916 64 bit (AMD64)]

    So when using functools.singledispatch or functools.singledispatchmethod, you need to provide at least 1 positional argument so it can dispatch based on the type of said argument

    however, with functools.singledispatchmethod, you get an error message that is confusing and not obvious what the problem is.

    Example with functools.singledispatchmethod:

    class NegatorTwo:
    @singledispatchmethod
    def neg(arg):
        raise NotImplementedError("Cannot negate a")

    @neg.register
    def _(self, arg: int):
        return -arg

    @neg.register
    def _test(self, arg: bool):
        return not arg


if __name__ == "__main__":

    n = NegatorTwo()
    print(n.neg(0))
    print(n.neg(False))
    print(n.neg(arg=0))

    you end up getting:

    PS C:\Users\mark> py -3 C:\Users\mark\Temp\singledisp.py
0
True
Traceback (most recent call last):
  File "C:\Users\mark\Temp\singledisp.py", line 58, in <module>
    print(n.neg(arg=0))
  File "C:\Python38\lib\functools.py", line 910, in _method
    method = self.dispatcher.dispatch(args[0].__class__)
IndexError: tuple index out of range

    but with just regular functools.singledispatch:

    
@functools.singledispatch
def negate_func(arg):
    raise NotImplementedError("can't negate")

@negate_func.register
def negate_int_func(arg:int):
    return -arg

@negate_func.register
def negate_bool_func(arg:bool):
    return not arg


if __name__ == "__main__":


    print(negate_func(0))
    print(negate_func(False))
    print(negate_func(arg=0))

    you get an error that tells you what actually is wrong:

    
PS C:\Users\mark> py -3 C:\Users\mark\Temp\singledisp.py
0
True
Traceback (most recent call last):
  File "C:\Users\mark\Temp\singledisp.py", line 63, in <module>
    print(negate_func(arg=0))
  File "C:\Python38\lib\functools.py", line 871, in wrapper
    raise TypeError(f'{funcname} requires at least '
TypeError: negate_func requires at least 1 positional argument

    it seems that the code in functools.singledispatchmethod needs to check to see if args is empty, and throw a similar (if not the same) exception as functools.singledispatch

    @mgrandi mgrandi mannequin added 3.8 only security fixes stdlib Python modules in the Lib dir type-bug An unexpected behavior, bug, or error labels Jun 26, 2020
    @mgrandi mgrandi mannequin changed the title functools.singledispatchfunction has confusing error message if no position arguments are passed in functools.singledispatchfunction has confusing error message if no positional arguments are passed in Jun 26, 2020
    @mgrandi mgrandi mannequin changed the title functools.singledispatchfunction has confusing error message if no position arguments are passed in functools.singledispatchfunction has confusing error message if no positional arguments are passed in Jun 26, 2020
    @AlexWaygood AlexWaygood added 3.9 only security fixes 3.10 only security fixes 3.11 only security fixes and removed 3.8 only security fixes labels Nov 5, 2021
    @ezio-melotti ezio-melotti transferred this issue from another repository Apr 10, 2022
    @andreamoro
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    andreamoro commented Nov 9, 2022 •
    edited

    Any progress with this, I apparently found this by chance after having spent a good hour to understand the missing positional argument was the cause of my troubles.

    I'm on Python 3.10.7

    @AlexWaygood AlexWaygood added 3.12 bugs and security fixes and removed 3.9 only security fixes labels Nov 9, 2022
    @skirpichev skirpichev mentioned this issue Feb 12, 2023
    Open
    @bedevere-app bedevere-app bot mentioned this issue Feb 16, 2024
    Merged
    serhiy-storchaka added a commit that referenced this issue Feb 16, 2024
    @ammaraskar @serhiy-storchaka
    gh-85294: Handle missing arguments to @singledispatchmethod gracefully (
    8b776e0 
    GH-21471)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
    @serhiy-storchaka serhiy-storchaka added 3.13 new features, bugs and security fixes and removed 3.11 only security fixes 3.10 only security fixes 3.12 bugs and security fixes labels Feb 16, 2024
    woodruffw pushed a commit to woodruffw-forks/cpython that referenced this issue Mar 4, 2024
    @ammaraskar @serhiy-storchaka @woodruffw
    pythongh-85294: Handle missing arguments to @singledispatchmethod gra…
    161661f 
    …cefully (pythonGH-21471)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
    diegorusso pushed a commit to diegorusso/cpython that referenced this issue Apr 17, 2024
    @ammaraskar @serhiy-storchaka @diegorusso
    pythongh-85294: Handle missing arguments to @singledispatchmethod gra…
    abbf6a4 
    …cefully (pythonGH-21471)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
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