Ignore AttributeError for missing __prepare__.

extra_tests/snippets/class.py

@@ -1,3 +1,4 @@
+from testutils import assert_raises
 __name__ = "class"
 
 
@@ -287,3 +288,12 @@ assert type(type.mro).__name__ == 'method_descriptor'
 
 B('x', (int,), {}).x()
 A()
+
+
+# Check that any appropriate callable can be used as metaclass key:
+meta = lambda *a, **k: type(*a, **k)
+class _(metaclass=meta): pass
+
+# check that non-callables don't fail on an absent prepare:
+with assert_raises(TypeError, _msg="'int' object not callable"):
+    class _(metaclass=20): pass

vm/src/stdlib/builtins.rs

@@ -872,13 +872,16 @@ mod builtins {
         let bases: PyObjectRef = bases.into();
 
         // Prepare uses full __getattribute__ resolution chain.
-        // TODO RUSTPYTHON: Doesn't do what we want all the time:
-        // e.g class M(metaclass=2): pass should fail with 'int not callable'.
-        let prepare = vm.get_attribute(metaclass.clone(), "__prepare__")?;
-        let namespace = vm.invoke(
-            &prepare,
-            FuncArgs::new(vec![name_obj.clone().into(), bases.clone()], kwargs.clone()),
-        )?;
+        // If an AttributeError was raised (None), return an empty dict,
+        // else invoke prepare callable.
+        let namespace = vm
+            .get_attribute_opt(metaclass.clone(), "__prepare__")?
+            .map_or(Ok(vm.ctx.new_dict().into()), |prepare| {
+                vm.invoke(
+                    &prepare,
+                    FuncArgs::new(vec![name_obj.clone().into(), bases.clone()], kwargs.clone()),
+                )
+            })?;
 
         // Accept any PyMapping as namespace.
         let namespace = PyMapping::try_from_object(vm, namespace.clone()).map_err(|_| {