bpo-44717: improve AttributeError on circular imports of submodules (GH-27299)

Signed-off-by: Filipe Laíns <lains@riseup.net>

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

1 files changed, 30 insertions, 0 deletions

diff --git a/Objects/moduleobject.c b/Objects/moduleobject.c
index ef39bde..1c37eb0 100644
--- a/Objects/moduleobject.c
+++ b/Objects/moduleobject.c
@@ -739,6 +739,30 @@ _PyModuleSpec_IsInitializing(PyObject *spec)
     return 0;
 }
 
+/* Check if the submodule name is in the "_uninitialized_submodules" attribute
+   of the module spec.
+ */
+int
+_PyModuleSpec_IsUninitializedSubmodule(PyObject *spec, PyObject *name)
+{
+    if (spec == NULL) {
+         return 0;
+    }
+
+    _Py_IDENTIFIER(_uninitialized_submodules);
+    PyObject *value = _PyObject_GetAttrId(spec, &PyId__uninitialized_submodules);
+    if (value == NULL) {
+        return 0;
+    }
+
+    int is_uninitialized = PySequence_Contains(value, name);
+    Py_DECREF(value);
+    if (is_uninitialized == -1) {
+        return 0;
+    }
+    return is_uninitialized;
+}
+
 static PyObject*
 module_getattro(PyModuleObject *m, PyObject *name)
 {
@@ -773,6 +797,12 @@ module_getattro(PyModuleObject *m, PyObject *name)
                             "(most likely due to a circular import)",
                             mod_name, name);
         }
+        else if (_PyModuleSpec_IsUninitializedSubmodule(spec, name)) {
+            PyErr_Format(PyExc_AttributeError,
+                            "cannot access submodule '%U' of module '%U' "
+                            "(most likely due to a circular import)",
+                            name, mod_name);
+        }
         else {
             PyErr_Format(PyExc_AttributeError,
                             "module '%U' has no attribute '%U'",