[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Łukasz Langa]

Łukasz Langa  added the comment:

Thanks for the 3.9 fix, Alex! ✨  ✨

--
resolution:  -> fixed
stage: patch review -> resolved
status: open -> closed

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Łukasz Langa]

Łukasz Langa  added the comment:


New changeset 97388c204b557f30e48a2b2ef826868702204cf2 by Alex Waygood in 
branch '3.9':
[3.9] bpo-39679: Fix `singledispatchmethod` `classmethod`/`staticmethod` bug 
(GH-29087)
https://github.com/python/cpython/commit/97388c204b557f30e48a2b2ef826868702204cf2


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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Alex Waygood]

Change by Alex Waygood :


--
pull_requests: +27353
pull_request: https://github.com/python/cpython/pull/29087

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Łukasz Langa]

Łukasz Langa  added the comment:

Thanks for the new tests, Alex. Not closing this just yet because maybe we find 
a way to fix it in 3.9.8+.

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Łukasz Langa]

Łukasz Langa  added the comment:


New changeset c15ba304f35362470e29ea5626fed28366bc9571 by Miss Islington (bot) 
in branch '3.10':
bpo-39679: Add tests for classmethod/staticmethod singledispatchmethods 
(GH-29034) (GH-29072)
https://github.com/python/cpython/commit/c15ba304f35362470e29ea5626fed28366bc9571


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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Łukasz Langa]

Łukasz Langa  added the comment:


New changeset ad6d162e518963711d24c80f1b7d6079bd437584 by Alex Waygood in 
branch 'main':
bpo-39679: Add tests for classmethod/staticmethod singledispatchmethods 
(GH-29034)
https://github.com/python/cpython/commit/ad6d162e518963711d24c80f1b7d6079bd437584


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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[miss-islington]

Change by miss-islington :


--
nosy: +miss-islington
nosy_count: 12.0 -> 13.0
pull_requests: +27340
pull_request: https://github.com/python/cpython/pull/29072

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Alex Waygood]

Change by Alex Waygood :


--
keywords: +patch
pull_requests: +27305
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/29034

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Alex Waygood]

Alex Waygood  added the comment:

Happily, this bug appears to have been resolved in Python 3.10 due to the fact 
that a `classmethod` wrapping a function `F` will now have an `__annotations__` 
dict equal to `F`.

In Python 3.9:

```
>>> x = lambda y: y
>>> x.__annotations__ = {'y': int}
>>> c = classmethod(x)
>>> c.__annotations__
Traceback (most recent call last):
  File "", line 1, in 
c.__annotations__
AttributeError: 'classmethod' object has no attribute '__annotations__'
```

In Python 3.10:

```
x = lambda y: y
x.__annotations__ = {'y': int}
c = classmethod(x)
c.__annotations__
{'y': }
```

This change appears to have resolved the bug in 
`functools.singledispatchmethod`. The bug remains in Python 3.9, however.

--
nosy: +AlexWaygood
versions:  -Python 3.8

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Dmitry Kulazhenko]

Change by Dmitry Kulazhenko :


--
nosy: +EugenePY, FFY00, Viktor Roytman, glyph, levkivskyi, lukasz.langa, 
markgrandi, mental, ncoghlan, xtreak

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Dmitry Kulazhenko]

Dmitry Kulazhenko  added the comment:

Based on what I've read, workaround:


from functools import singledispatchmethod


def _register(self, cls, method=None):
if hasattr(cls, "__func__"):
setattr(cls, "__annotations__", cls.__func__.__annotations__)
return self.dispatcher.register(cls, func=method)


singledispatchmethod.register = _register

--
nosy: +dmkulazhenko -EugenePY, FFY00, Viktor Roytman, glyph, levkivskyi, 
lukasz.langa, markgrandi, mental, ncoghlan, xtreak

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[mental]

Change by mental :


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nosy: +mental

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Eugene-Yuan Kou]

Eugene-Yuan Kou  added the comment:

Hi, I also encounter to the problem, and I have give my attempt to make the 
singledispatchmethod support the classmethod, and staticmethod with type 
annotation. I also adding two tests. Please refer to my fork.  I will trying to 
make a pull request

https://github.com/EugenePY/cpython/compare/3.8...fix-issue-39679

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Filipe Laíns]

Change by Filipe Laíns :


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nosy: +FFY00

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Glyph Lefkowitz]

Glyph Lefkowitz  added the comment:

I also just discovered this.  I thought it must have regressed at some point 
but I see the docs say "new in 3.8" and I'm using 3.8.

Is there a "no CI for examples in the docs" issue that this could be linked to?

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Mark Grandi]

Mark Grandi  added the comment:

same issue here, if we can't fix it then maybe we should edit the documentation 
to not suggest that @classmethod works?

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Viktor Roytman]

Viktor Roytman  added the comment:

Sorry to bump this after months of inactivity but is there something I need to 
do here?

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

Sorry, I had the part only to detect annotations attached. My part is something 
similar to yours except it stores the appropriate function in the registry 
itself instead of passing the arguments at __get__ .

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Viktor Roytman]

Viktor Roytman  added the comment:

I tried to apply this change but it didn't work, failing with this error

$ ~/.pyenv/versions/3.8.1/bin/python -m bad_classmethod_as_documented
Traceback (most recent call last):
  File "/home/viktor/.pyenv/versions/3.8.1/lib/python3.8/runpy.py", line 
193, in _run_module_as_main
return _run_code(code, main_globals, None,
  File "/home/viktor/.pyenv/versions/3.8.1/lib/python3.8/runpy.py", line 
86, in _run_code
exec(code, run_globals)
  File "/home/viktor/scratch/bad_classmethod_as_documented.py", line 4, in 

class Negator:
  File "/home/viktor/scratch/bad_classmethod_as_documented.py", line 12, in 
Negator
def _(cls, arg: int):
  File "/home/viktor/.pyenv/versions/3.8.1/lib/python3.8/functools.py", 
line 1006, in register
return self.dispatcher.register(cls, func=method)
  File "/home/viktor/.pyenv/versions/3.8.1/lib/python3.8/functools.py", 
line 959, in register
argname, cls = next(iter(get_type_hints(func).items()))
  File "/home/viktor/.pyenv/versions/3.8.1/lib/python3.8/typing.py", line 
1252, in get_type_hints
raise TypeError('{!r} is not a module, class, method, '
TypeError:  is not a module, class, 
method, or function.

After digging around a bit, this diff seems to work (not sure if there's a 
better way to do it) (also this one doesn't seem to care whether @staticmethod 
is applied to the implementation methods):

$ diff -u functools-orig.py functools.py
--- functools-orig.py   2020-02-21 16:14:56.141934001 -0500
+++ functools.py2020-02-21 16:17:19.959905849 -0500
@@ -843,14 +843,18 @@
 if func is None:
 if isinstance(cls, type):
 return lambda f: register(cls, f)
-ann = getattr(cls, '__annotations__', {})
+if isinstance(cls, (classmethod, staticmethod)):
+ann = getattr(cls.__func__, '__annotations__', {})
+func = cls.__func__
+else:
+ann = getattr(cls, '__annotations__', {})
+func = cls
 if not ann:
 raise TypeError(
 f"Invalid first argument to `register()`: {cls!r}. "
 f"Use either `@register(some_class)` or plain `@register` "
 f"on an annotated function."
 )
-func = cls
 
 # only import typing if annotation parsing is necessary
 from typing import get_type_hints
@@ -908,6 +912,8 @@
 def __get__(self, obj, cls=None):
 def _method(*args, **kwargs):
 method = self.dispatcher.dispatch(args[0].__class__)
+if isinstance(self.func, classmethod):
+return method.__get__(obj, cls)(cls, *args, **kwargs)
 return method.__get__(obj, cls)(*args, **kwargs)
 
 _method.__isabstractmethod__ = self.__isabstractmethod__

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan  added the comment:

I guess the method checks for annotation on cls [0] which will be 
classmethod/staticmethod object in the report and won't have annotations. The 
annotations should be looked up in the function the classmethod/staticmethod 
decorator wraps around as in cls.__func__ . Something like below so that the 
right annotations are picked up. In addition to this the registry should store 
the type annotation as key to cls or cls.__func__ depending on normal method or 
classmethod/staticmethod.

diff --git Lib/functools.py Lib/functools.py
index 050bec8605..a66711208d 100644
--- Lib/functools.py
+++ Lib/functools.py
@@ -1073,24 +1073,33 @@ def singledispatch(func):
 if func is None:
 if isinstance(cls, type):
 return lambda f: register(cls, f)
-ann = getattr(cls, '__annotations__', {})
+if isinstance(cls, (classmethod, staticmethod)):
+ann = getattr(cls.__func__, '__annotations__', {})
+func = cls.__func__
+else:
+ann = getattr(cls, '__annotations__', {})
+func = cls

[0] 
https://github.com/python/cpython/blob/ab6423fe2de0ed5f8a0dc86a9c7070229326b0f0/Lib/functools.py#L1076

--
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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan :


--
nosy: +lukasz.langa

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[issue39679] functools: singledispatchmethod doesn't work with classmethod

[Viktor Roytman]

New submission from Viktor Roytman :

I couldn't get the example given for the interaction between 
@singledispatchmethod and @classmethod to work 
https://docs.python.org/3/library/functools.html?highlight=singledispatch#functools.singledispatchmethod

from functools import singledispatchmethod


class Negator:
@singledispatchmethod
@classmethod
def neg(cls, arg):
raise NotImplementedError("Cannot negate a")

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

@neg.register
@classmethod
def _(cls, arg: bool):
return not arg


if __name__ == "__main__":
print(Negator.neg(0))
print(Negator.neg(False))

Leads to

$ python -m bad_classmethod_as_documented
Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 193, in _run_module_as_main
return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 86, in _run_code
exec(code, run_globals)
  File "/home/viktor/scratch/bad_classmethod_as_documented.py", line 4, in 

class Negator:
  File "/home/viktor/scratch/bad_classmethod_as_documented.py", line 12, in 
Negator
def _(cls, arg: int):
  File "/usr/lib/python3.8/functools.py", line 906, in register
return self.dispatcher.register(cls, func=method)
  File "/usr/lib/python3.8/functools.py", line 848, in register
raise TypeError(
TypeError: Invalid first argument to `register()`: . Use either `@register(some_class)` or plain `@register` on an 
annotated function.

Curiously, @staticmethod does work, but not as documented (don't decorate the 
actual implementations):

from functools import singledispatchmethod


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

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

@neg.register
def _(arg: bool):
return not arg


if __name__ == "__main__":
print(Negator.neg(0))
print(Negator.neg(False))

Leads to

$ python -m good_staticmethod
0
True

Removing @classmethod from the implementation methods doesn't work, though

Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 193, in _run_module_as_main
return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 86, in _run_code
exec(code, run_globals)
  File "/home/viktor/scratch/bad_classmethod_alternative.py", line 20, in 

print(Negator.neg(0))
  File "/usr/lib/python3.8/functools.py", line 911, in _method
return method.__get__(obj, cls)(*args, **kwargs)
TypeError: _() missing 1 required positional argument: 'arg'

--
components: Library (Lib)
messages: 362233
nosy: Viktor Roytman
priority: normal
severity: normal
status: open
title: functools: singledispatchmethod doesn't work with classmethod
type: behavior
versions: Python 3.8, Python 3.9

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