[3.9] bpo-42073: allow classmethod to wrap other classmethod-like descriptors. #22757
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bpo-19072 (python#8405) allows `classmethod` to wrap other descriptors, but this does not work when the wrapped descriptor mimics classmethod. The current PR fixes this. In Python 3.8 and before, one could create a callable descriptor such that this works as expected (see Lib/test/test_decorators.py for examples): ```python class A: @myclassmethod def f1(cls): return cls @classmethod @myclassmethod def f2(cls): return cls ``` In Python 3.8 and before, `A.f2()` return `A`. Currently in Python 3.9, it returns `type(A)`. This PR make `A.f2()` return `A` again. As of python#8405, classmethod calls `obj.__get__(type)` if `obj` has `__get__`. This allows one to chain `@classmethod` and `@property` together. When using classmethod-like descriptors, it's the second argument to `__get__`--the owner or the type--that is important, but this argument is currently missing. Since it is None, the "owner" argument is assumed to be the type of the first argument, which, in this case, is wrong (we want `A`, not `type(A)`). This PR updates classmethod to call `obj.__get__(type, type)` if `obj` has `__get__`.
eriknw changed the title 
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ambv requested review from 1st1, abalkin, asvetlov, berkerpeksag, brettcannon, encukou, ericsnowcurrently, ericvsmith, ethanfurman, gpshead, gvanrossum, ilevkivskyi, isidentical, lysnikolaou, markshannon, methane, ncoghlan, pablogsal, pganssle, rhettinger, terryjreedy and tiran as code owners 
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| Oops, changing the base of the pull request didn't do what I thought it would and reverting back to 3.9 leaves the mess as is. I'll recreate this pull request on |
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bpo-19072 (#8405) allows
classmethodto wrap other descriptors, but this does not work when the wrapped descriptor mimics classmethod. The current PR fixes this.In Python 3.8 and before, one could create a callable descriptor such that this works as expected (see Lib/test/test_decorators.py for examples):
In Python 3.8 and before,
A.f2()returnA. Currently in Python 3.9, it returnstype(A). This PR makeA.f2()returnAagain.As of #8405, classmethod calls
obj.__get__(type)ifobjhas__get__. This allows one to chain@classmethodand@propertytogether. When using classmethod-like descriptors, it's the second argument to__get__--the owner or the type--that is important, but this argument is currently missing. Since it is None, the "owner" argument is assumed to be the type of the first argument, which, in this case, is wrong (we wantA, nottype(A)).This PR updates classmethod to call
obj.__get__(type, type)ifobjhas__get__.This PR is targeting Python 3.9 branch, because I think this is a bug fix. I can also target master (3.10).
I'm not really sure where to add a note of this change.
https://bugs.python.org/issue42073