Fix classmethod descr_get to match CPython behavior (RustPython#7420)

ever0de · youknowone · commit 151ba9a527d5 · 2026-03-20T05:01:20.000+09:00
Simplify classmethod.__get__ to always create a PyBoundMethod binding
  the callable to the class, matching CPython's cm_descr_get which simply
  calls PyMethod_New(cm-&gt;cm_callable, type).

  The previous implementation incorrectly tried to call __get__ on the
  wrapped callable, which broke when a bound method was passed to
  classmethod() (e.g. classmethod(A().foo)).
diff --git a/Lib/test/test_decorators.py b/Lib/test/test_decorators.py
@@ -291,7 +291,6 @@ def bar(): return 42
         self.assertEqual(bar(), 42)
         self.assertEqual(actions, expected_actions)
 
-    @unittest.expectedFailure # TODO: RUSTPYTHON
     def test_bound_function_inside_classmethod(self):
         class A:
             def foo(self, cls):
diff --git a/crates/vm/src/builtins/classmethod.rs b/crates/vm/src/builtins/classmethod.rs
@@ -57,13 +57,8 @@ impl GetDescriptor for PyClassMethod {
     ) -> PyResult {
         let (zelf, _obj) = Self::_unwrap(&zelf, obj, vm)?;
         let cls = cls.unwrap_or_else(|| _obj.class().to_owned().into());
-        // Clone and release lock before calling Python code to prevent deadlock
         let callable = zelf.callable.lock().clone();
-        let call_descr_get: PyResult<PyObjectRef> = callable.get_attr("__get__", vm);
-        match call_descr_get {
-            Err(_) => Ok(PyBoundMethod::new(cls, callable).into_ref(&vm.ctx).into()),
-            Ok(call_descr_get) => call_descr_get.call((cls.clone(), cls), vm),
-        }
+        Ok(PyBoundMethod::new(cls, callable).into_ref(&vm.ctx).into())
     }
 }
 

Original file line number	Diff line number	Diff line change
`@@ -57,13 +57,8 @@ impl GetDescriptor for PyClassMethod {`
`57`	`57`	`) -> PyResult {`
`58`	`58`	`let (zelf, _obj) = Self::_unwrap(&zelf, obj, vm)?;`
`59`	`59`	`let cls = cls.unwrap_or_else(\|\| _obj.class().to_owned().into());`
`60`		`- // Clone and release lock before calling Python code to prevent deadlock`
`61`	`60`	`let callable = zelf.callable.lock().clone();`
`62`		`- let call_descr_get: PyResult<PyObjectRef> = callable.get_attr("__get__", vm);`
`63`		`- match call_descr_get {`
`64`		`- Err(_) => Ok(PyBoundMethod::new(cls, callable).into_ref(&vm.ctx).into()),`
`65`		`- Ok(call_descr_get) => call_descr_get.call((cls.clone(), cls), vm),`
`66`		`- }`
	`61`	`+ Ok(PyBoundMethod::new(cls, callable).into_ref(&vm.ctx).into())`
`67`	`62`	`}`
`68`	`63`	`}`
`69`	`64`