[mypyc] Fix AttributeError message (#13382)

Previously this would be `attribute 'ClsName' of 'attr_name' undefined`.
Now it's `attribute 'attr_name' of 'ClsName' undefined`.

Author: JukkaL

mypyc/lib-rt/exc_ops.c

@@ -243,7 +243,7 @@ void CPy_TypeErrorTraceback(const char *filename, const char *funcname, int line
 void CPy_AttributeError(const char *filename, const char *funcname, const char *classname,
                         const char *attrname, int line, PyObject *globals) {
     char buf[500];
-    snprintf(buf, sizeof(buf), "attribute '%.200s' of '%.200s' undefined", classname, attrname);
+    snprintf(buf, sizeof(buf), "attribute '%.200s' of '%.200s' undefined", attrname, classname);
     PyErr_SetString(PyExc_AttributeError, buf);
     CPy_AddTraceback(filename, funcname, line, globals);
 }

mypyc/test-data/run-classes.test

@@ -78,9 +78,9 @@ def test_delete() -> None:
     del c.x
     del c.y
     assert c.z == 3
-    with assertRaises(AttributeError):
+    with assertRaises(AttributeError, "attribute 'x' of 'C' undefined"):
         c.x
-    with assertRaises(AttributeError):
+    with assertRaises(AttributeError, "attribute 'y' of 'C' undefined"):
         c.y
 
 def test_delete_any() -> None: