bpo-33237: Improve AttributeError message for partially initialized module. (GH-6398)

1 files changed, 39 insertions, 2 deletions

diff --git a/Objects/moduleobject.c b/Objects/moduleobject.c
index ccf5f8e..5181941 100644
--- a/Objects/moduleobject.c
+++ b/Objects/moduleobject.c
@@ -698,6 +698,27 @@ module_repr(PyModuleObject *m)
     return PyObject_CallMethod(interp->importlib, "_module_repr", "O", m);
 }
 
+/* Check if the "_initializing" attribute of the module spec is set to true.
+   Clear the exception and return 0 if spec is NULL.
+ */
+int
+_PyModuleSpec_IsInitializing(PyObject *spec)
+{
+    if (spec != NULL) {
+        _Py_IDENTIFIER(_initializing);
+        PyObject *value = _PyObject_GetAttrId(spec, &PyId__initializing);
+        if (value != NULL) {
+            int initializing = PyObject_IsTrue(value);
+            Py_DECREF(value);
+            if (initializing >= 0) {
+                return initializing;
+            }
+        }
+    }
+    PyErr_Clear();
+    return 0;
+}
+
 static PyObject*
 module_getattro(PyModuleObject *m, PyObject *name)
 {
@@ -717,8 +738,24 @@ module_getattro(PyModuleObject *m, PyObject *name)
         _Py_IDENTIFIER(__name__);
         mod_name = _PyDict_GetItemId(m->md_dict, &PyId___name__);
         if (mod_name && PyUnicode_Check(mod_name)) {
-            PyErr_Format(PyExc_AttributeError,
-                        "module '%U' has no attribute '%U'", mod_name, name);
+            _Py_IDENTIFIER(__spec__);
+            Py_INCREF(mod_name);
+            PyObject *spec = _PyDict_GetItemId(m->md_dict, &PyId___spec__);
+            Py_XINCREF(spec);
+            if (_PyModuleSpec_IsInitializing(spec)) {
+                PyErr_Format(PyExc_AttributeError,
+                             "partially initialized "
+                             "module '%U' has no attribute '%U' "
+                             "(most likely due to a circular import)",
+                             mod_name, name);
+            }
+            else {
+                PyErr_Format(PyExc_AttributeError,
+                             "module '%U' has no attribute '%U'",
+                             mod_name, name);
+            }
+            Py_XDECREF(spec);
+            Py_DECREF(mod_name);
             return NULL;
         }
     }