bpo-42073: allow classmethod to wrap other classmethod-like descriptors.

bpo-19072 (#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 #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 · Oct 18, 2020 · Oct 18, 2020 · f210e7f15e1367614bf3d5acf3bdeae949cd89f4
commit f210e7f15e1367614bf3d5acf3bdeae949cd89f4
diff --git a/Lib/test/test_decorators.py b/Lib/test/test_decorators.py
@@ -1,4 +1,5 @@
 import unittest
+from types import MethodType
 
 def funcattrs(**kwds):
     def decorate(func):
@@ -307,6 +308,91 @@ def outer(cls):
         self.assertEqual(Class().inner(), 'spam')
         self.assertEqual(Class().outer(), 'eggs')
 
+    def test_wrapped_classmethod_inside_classmethod(self):
+        class MyClassMethod1:
+            def __init__(self, func):
+                self.func = func
+
+            def __call__(self, cls):
+                if hasattr(self.func, '__get__'):
+                    return self.func.__get__(cls, cls)()
+                return self.func(cls)
+
+            def __get__(self, instance, owner=None):
+                if owner is None:
+                    owner = type(instance)
+                return MethodType(self, owner)
+
+        class MyClassMethod2:
+            def __init__(self, func):
+                if isinstance(func, classmethod):
+                    func = func.__func__
+                self.func = func
+
+            def __call__(self, cls):
+                return self.func(cls)
+
+            def __get__(self, instance, owner=None):
+                if owner is None:
+                    owner = type(instance)
+                return MethodType(self, owner)
+
+        for myclassmethod in [MyClassMethod1, MyClassMethod2]:
+            class A:
+                @myclassmethod
+                def f1(cls):
+                    return cls
+
+                @classmethod
+                @myclassmethod
+                def f2(cls):
+                    return cls
+
+                @myclassmethod
+                @classmethod
+                def f3(cls):
+                    return cls
+
+                @classmethod
+                @classmethod
+                def f4(cls):
+                    return cls
+
+                @myclassmethod
+                @MyClassMethod1
+                def f5(cls):
+                    return cls
+
+                @myclassmethod
+                @MyClassMethod2
+                def f6(cls):
+                    return cls
+
+            self.assertIs(A.f1(), A)
+            self.assertIs(A.f2(), A)
+            self.assertIs(A.f3(), A)
+            self.assertIs(A.f4(), A)
+            self.assertIs(A.f5(), A)
+            self.assertIs(A.f6(), A)
+            a = A()
+            self.assertIs(a.f1(), A)
+            self.assertIs(a.f2(), A)
+            self.assertIs(a.f3(), A)
+            self.assertIs(a.f4(), A)
+            self.assertIs(a.f5(), A)
+            self.assertIs(a.f6(), A)
+
+            def f(cls):
+                return cls
+
+            self.assertIs(myclassmethod(f).__get__(a)(), A)
+            self.assertIs(myclassmethod(f).__get__(a, A)(), A)
+            self.assertIs(myclassmethod(f).__get__(A, A)(), A)
+            self.assertIs(myclassmethod(f).__get__(A)(), type(A))
+            self.assertIs(classmethod(f).__get__(a)(), A)
+            self.assertIs(classmethod(f).__get__(a, A)(), A)
+            self.assertIs(classmethod(f).__get__(A, A)(), A)
+            self.assertIs(classmethod(f).__get__(A)(), type(A))
 
 class TestClassDecorators(unittest.TestCase):
 

diff --git a/Objects/funcobject.c b/Objects/funcobject.c
@@ -739,7 +739,7 @@ cm_descr_get(PyObject *self, PyObject *obj, PyObject *type)
         type = (PyObject *)(Py_TYPE(obj));
     if (Py_TYPE(cm->cm_callable)->tp_descr_get != NULL) {
         return Py_TYPE(cm->cm_callable)->tp_descr_get(cm->cm_callable, type,
-                                                      NULL);
+                                                      type);
     }
     return PyMethod_New(cm->cm_callable, type);
 }