New submission from Viktor Roytman <viktor.royt...@gmail.com>:
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 <module> 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()`: <classmethod object at 0x7f37d1469070>. 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 <module> 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 _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue39679> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
