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Commit2ce8af3

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bpo-42073: allow classmethod to wrap other classmethod-like descriptors (GH-27115) (GH-27162)
Patch by Erik Welch.bpo-19072 (GH-8405) allows `classmethod` to wrap other descriptors, but this doesnot work when the wrapped descriptor mimics classmethod. The current PR fixesthis.In Python 3.8 and before, one could create a callable descriptor such that thisworks as expected (see Lib/test/test_decorators.py for examples):```pythonclass 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, itreturns `type(A)`. This PR make `A.f2()` return `A` again.As ofGH-8405, classmethod calls `obj.__get__(type)` if `obj` has `__get__`.This allows one to chain `@classmethod` and `@property` together. Whenusing classmethod-like descriptors, it's the second argument to `__get__`--theowner 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 firstargument, 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__`.Co-authored-by: Erik Welch <erik.n.welch@gmail.com>(cherry picked from commitb83861f)
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3 files changed

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‎Lib/test/test_decorators.py

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fromtestimportsupport
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importunittest
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fromtypesimportMethodType
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deffuncattrs(**kwds):
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defdecorate(func):
@@ -329,6 +330,91 @@ def outer(cls):
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self.assertEqual(Class().inner(),'spam')
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self.assertEqual(Class().outer(),'eggs')
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deftest_wrapped_classmethod_inside_classmethod(self):
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classMyClassMethod1:
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def__init__(self,func):
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self.func=func
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def__call__(self,cls):
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ifhasattr(self.func,'__get__'):
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returnself.func.__get__(cls,cls)()
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returnself.func(cls)
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def__get__(self,instance,owner=None):
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ifownerisNone:
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owner=type(instance)
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returnMethodType(self,owner)
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classMyClassMethod2:
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def__init__(self,func):
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ifisinstance(func,classmethod):
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func=func.__func__
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self.func=func
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def__call__(self,cls):
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returnself.func(cls)
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def__get__(self,instance,owner=None):
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ifownerisNone:
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owner=type(instance)
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returnMethodType(self,owner)
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formyclassmethodin [MyClassMethod1,MyClassMethod2]:
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classA:
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@myclassmethod
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deff1(cls):
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returncls
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@classmethod
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@myclassmethod
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deff2(cls):
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returncls
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@myclassmethod
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@classmethod
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deff3(cls):
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returncls
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@classmethod
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@classmethod
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deff4(cls):
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returncls
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@myclassmethod
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@MyClassMethod1
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deff5(cls):
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returncls
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@myclassmethod
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@MyClassMethod2
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deff6(cls):
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returncls
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self.assertIs(A.f1(),A)
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self.assertIs(A.f2(),A)
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self.assertIs(A.f3(),A)
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self.assertIs(A.f4(),A)
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self.assertIs(A.f5(),A)
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self.assertIs(A.f6(),A)
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a=A()
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self.assertIs(a.f1(),A)
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self.assertIs(a.f2(),A)
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self.assertIs(a.f3(),A)
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self.assertIs(a.f4(),A)
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self.assertIs(a.f5(),A)
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self.assertIs(a.f6(),A)
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deff(cls):
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returncls
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self.assertIs(myclassmethod(f).__get__(a)(),A)
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self.assertIs(myclassmethod(f).__get__(a,A)(),A)
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self.assertIs(myclassmethod(f).__get__(A,A)(),A)
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self.assertIs(myclassmethod(f).__get__(A)(),type(A))
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self.assertIs(classmethod(f).__get__(a)(),A)
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self.assertIs(classmethod(f).__get__(a,A)(),A)
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self.assertIs(classmethod(f).__get__(A,A)(),A)
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self.assertIs(classmethod(f).__get__(A)(),type(A))
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classTestClassDecorators(unittest.TestCase):
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The ``@classmethod`` decorator can now wrap other classmethod-like
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descriptors.

‎Objects/funcobject.c

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@@ -825,7 +825,7 @@ cm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
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type= (PyObject*)(Py_TYPE(obj));
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if (Py_TYPE(cm->cm_callable)->tp_descr_get!=NULL) {
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returnPy_TYPE(cm->cm_callable)->tp_descr_get(cm->cm_callable,type,
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NULL);
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type);
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}
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returnPyMethod_New(cm->cm_callable,type);
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}

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