Description

assignee = None
closed_at = None
created_at = <Date 2020-05-01.07:04:01.217>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = "functools.singledispatch doesn't verify annotation is on FIRST parameter"
updated_at = <Date 2021-11-09.22:47:09.886>
user = 'https://github.com/Dutcho'

activity = <Date 2021-11-09.22:47:09.886>
actor = 'AlexWaygood'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2020-05-01.07:04:01.217>
creator = 'Dutcho'
dependencies = []
files = []
hgrepos = []
issue_num = 40464
keywords = ['patch']
message_count = 4.0
messages = ['367824', '368079', '368095', '406062']
nosy_count = 6.0
nosy_names = ['rhettinger', 'lukasz.langa', 'Dutcho', 'FFY00', 'bim_bam', 'AlexWaygood']
pr_nums = ['19871']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40464'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

Activity

1. Dutcho commented on May 1, 2020

   

   Dutchomannequin

   on May 1, 2020

   MannequinAuthor

   More actions

   From Python 3.7, functools.singledispatch makes the register() attribute of the generic function infer the type of the first argument automatically for functions annotated with types. That's great for DRY.
   However, in 3.7 and 3.8, no check is made that the first parameter of the registered function is actually annotated; any annotation suffices, even the return one.

   Example:

   >>> @functools.singledispatch
   ... def func(arg):...
   >>> @func.register
   ... def _int(arg) -> int:...
   >>> @func.register
   ... def _str(arg) -> str:...
   

   No errors happen, although the return type, not arg, is annotated.
   This results in:

   >>> func.registry
   mappingproxy({<class 'object'>: <function func>, <class 'int'>: <function _int>, <class 'str'>: <function _str>})
   

   Obviously, that doesn't dispatch correctly.

   Note that un-annotated functions *are* caught:

   >>> @func.register
   ... def _no_annotation(arg): ...
   Traceback (most recent call last):
   ...
   TypeError: Invalid first argument to `register()`: <function _no_annotation at 0x000001D769A43D30>. Use either `@register(some_class)` or plain `@register` on an annotated function.
   
2. Dutcho

   added

   3.7 (EOL)end of lifeend of life

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on May 1, 2020
3. Dutcho commented on May 4, 2020

   

   Dutchomannequin

   on May 4, 2020

   MannequinAuthor

   More actions

   I'm afraid my "even return" was interpreted in #19871 as "only return", while as stated "any annotation" suffices. To rephrase:
   If the *first* parameter of the registered function isn't annotated, any non-first annotation suffices for registering, but will not dispatch correctly.
   Example:

   >>> @functools.singledispatch
   ... def func(arg, x):...
   >>> @func.register
   ... def _int(arg, x:int):...
   >>> @func.register
   ... def _str(arg, x:str):...
   

   No errors happen, although parameter x is annotated, not the first parameter arg. So func() will dispatch on the type of arg according to the annotation of x.
   So I'm afraid the PR solves the specific "return" example case, but not the flagged general issue.
4. FFY00 commented on May 5, 2020

   

   FFY00

   on May 5, 2020

   Member

   More actions

   Right, forgot about that. We can get the first argument name from inspect.signature and then fetch it from the get_type_hints dictionary, I don't know a better way to do it.
5. AlexWaygood commented on Nov 9, 2021

   

   AlexWaygood

   on Nov 9, 2021

   Member

   More actions

   Reproduced on 3.11.
6. AlexWaygood

   added

   3.9 (EOL)end of lifeend of life

   3.10 (EOL)end of lifeend of life

   3.11only security fixesonly security fixes

   and removed

   3.7 (EOL)end of lifeend of life

   on Nov 9, 2021
7. 7 remaining items
8. ZeroIntensity

   added

   3.13only security fixesonly security fixes

   3.14bugs and security fixesbugs and security fixes

   3.15bugs and security fixesbugs and security fixes

   and removed

   3.9 (EOL)end of lifeend of life

   on Oct 17, 2025
9. johnslavik

   added 3 commits that reference this issue on Jan 7, 2026

   Add precedent case for pythonGH-84644

   ec01821

   Fix pythonGH-84644 test

   57faa34

   Fix `TestSingleDispatch.test_method_signatures` to put annotations on…

   30a5245
10. serhiy-storchaka

    added a commit that references this issue on Jan 11, 2026

    gh-127750: Fix annotations in singledispatchmethod signature tests (G…

    620a5b9
11. miss-islington

    added 2 commits that reference this issue on Jan 11, 2026

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    9c7f1fc

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    c9eccc7
12. serhiy-storchaka

    added 2 commits that reference this issue on Jan 11, 2026

    [3.13] gh-127750: Fix annotations in singledispatchmethod signature t…

    ea1ecf6

    [3.14] gh-127750: Fix annotations in singledispatchmethod signature t…

    ea26c88
13. reidenong

    added a commit that references this issue on Jan 12, 2026

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    13c9f56
14. thunder-coding

    added a commit that references this issue on Feb 15, 2026

    pythongh-127750: Fix annotations in singledispatchmethod signature te…

    5b0a063
15. encukou

    added a commit that references this issue on Oct 21, 2026

    gh-84644: Deprecate singledispatch on return annotation (GH-143465)

    641ef0a

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