bpo-33237: Improve AttributeError message for partially initialized module.

Include/moduleobject.h

@@ -30,6 +30,7 @@ PyAPI_FUNC(PyObject *) PyModule_GetFilenameObject(PyObject *);
 #ifndef Py_LIMITED_API
 PyAPI_FUNC(void) _PyModule_Clear(PyObject *);
 PyAPI_FUNC(void) _PyModule_ClearDict(PyObject *);
+PyAPI_FUNC(int) _PyModuleSpec_IsInitializing(PyObject *);
 #endif
 PyAPI_FUNC(struct PyModuleDef*) PyModule_GetDef(PyObject*);
 PyAPI_FUNC(void*) PyModule_GetState(PyObject*);

Lib/test/test_import/__init__.py

@@ -1271,6 +1271,19 @@ def test_binding(self):
  except ImportError:
  self.fail('circular import with binding a submodule to a name failed')
 
+ def test_crossreference1(self):
+ import test.test_import.data.circular_imports.use
+ import test.test_import.data.circular_imports.source
+
+ def test_crossreference2(self):
+ with self.assertRaises(AttributeError) as cm:
+ import test.test_import.data.circular_imports.source
+ errmsg = str(cm.exception)
+ self.assertIn('test.test_import.data.circular_imports.source', errmsg)
+ self.assertIn('spam', errmsg)
+ self.assertIn('partially initialized module', errmsg)
+ self.assertIn('circular import', errmsg)
+
 
 if __name__ == '__main__':
  # Test needs to be a package, so we can do relative imports.

Lib/test/test_import/data/circular_imports/source.py

@@ -0,0 +1,2 @@
+from . import use
+spam = 1

Lib/test/test_import/data/circular_imports/use.py

@@ -0,0 +1,2 @@
+from . import source
+source.spam

Misc/NEWS.d/next/Core and Builtins/2018-07-24-12-54-57.bpo-33237.O95mps.rst

@@ -0,0 +1 @@
+Improved :exc:`AttributeError` message for partially initialized module.

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;
  }
  }

Python/import.c

@@ -1721,38 +1721,26 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *globals,
  mod = PyImport_GetModule(abs_name);
  if (mod != NULL && mod != Py_None) {
  _Py_IDENTIFIER(__spec__);
- _Py_IDENTIFIER(_initializing);
  _Py_IDENTIFIER(_lock_unlock_module);
- PyObject *value = NULL;
  PyObject *spec;
- int initializing = 0;
 
  /* Optimization: only call _bootstrap._lock_unlock_module() if
  __spec__._initializing is true.
  NOTE: because of this, initializing must be set *before*
  stuffing the new module in sys.modules.
  */
  spec = _PyObject_GetAttrId(mod, &PyId___spec__);
- if (spec != NULL) {
- value = _PyObject_GetAttrId(spec, &PyId__initializing);
- Py_DECREF(spec);
- }
- if (value == NULL)
- PyErr_Clear();
- else {
- initializing = PyObject_IsTrue(value);
- Py_DECREF(value);
- if (initializing == -1)
- PyErr_Clear();
- if (initializing > 0) {
- value = _PyObject_CallMethodIdObjArgs(interp->importlib,
- &PyId__lock_unlock_module, abs_name,
- NULL);
- if (value == NULL)
- goto error;
- Py_DECREF(value);
+ if (_PyModuleSpec_IsInitializing(spec)) {
+ PyObject *value = _PyObject_CallMethodIdObjArgs(interp->importlib,
+ &PyId__lock_unlock_module, abs_name,
+ NULL);
+ if (value == NULL) {
+ Py_DECREF(spec);
+ goto error;
  }
+ Py_DECREF(value);
  }
+ Py_XDECREF(spec);
  }
  else {
  Py_XDECREF(mod);