Issue #15767: Touch up ModuleNotFoundError usage by import.

Forgot to raise ModuleNotFoundError when None is found in sys.modules.
This led to introducing the C function PyErr_SetImportErrorSubclass()
to make setting ModuleNotFoundError easier.

Also updated the reference docs to mention ModuleNotFoundError
appropriately. Updated the docs for ModuleNotFoundError to mention the
None in sys.modules case.

Lastly, it was noticed that PyErr_SetImportError() was not setting an
exception when returning None in one case. That issue is now fixed.

3 files changed, 26 insertions, 6 deletions

diff --git a/Python/errors.c b/Python/errors.c
index 1f955b54f0..89021aadd6 100644
--- a/Python/errors.c
+++ b/Python/errors.c
@@ -619,12 +619,25 @@ PyObject *PyErr_SetFromWindowsErrWithUnicodeFilename(
 #endif /* MS_WINDOWS */
 
 PyObject *
-PyErr_SetImportError(PyObject *msg, PyObject *name, PyObject *path)
+PyErr_SetImportErrorSubclass(PyObject *exception, PyObject *msg,
+    PyObject *name, PyObject *path)
 {
+    int issubclass;
     PyObject *args, *kwargs, *error;
 
-    if (msg == NULL)
+    issubclass = PyObject_IsSubclass(exception, PyExc_ImportError);
+    if (issubclass < 0) {
+        return NULL;
+    }
+    else if (!issubclass) {
+        PyErr_SetString(PyExc_TypeError, "expected a subclass of ImportError");
+        return NULL;
+    }
+
+    if (msg == NULL) {
+        PyErr_SetString(PyExc_TypeError, "expected a message argument");
         return NULL;
+    }
 
     args = PyTuple_New(1);
     if (args == NULL)
@@ -649,7 +662,7 @@ PyErr_SetImportError(PyObject *msg, PyObject *name, PyObject *path)
     PyDict_SetItemString(kwargs, "name", name);
     PyDict_SetItemString(kwargs, "path", path);
 
-    error = PyObject_Call(PyExc_ImportError, args, kwargs);
+    error = PyObject_Call(exception, args, kwargs);
     if (error != NULL) {
         PyErr_SetObject((PyObject *)Py_TYPE(error), error);
         Py_DECREF(error);
@@ -661,6 +674,12 @@ PyErr_SetImportError(PyObject *msg, PyObject *name, PyObject *path)
     return NULL;
 }
 
+PyObject *
+PyErr_SetImportError(PyObject *msg, PyObject *name, PyObject *path)
+{
+    return PyErr_SetImportErrorSubclass(PyExc_ImportError, msg, name, path);
+}
+
 void
 _PyErr_BadInternalCall(const char *filename, int lineno)
 {
diff --git a/Python/import.c b/Python/import.c
index 0bb46d2e1e..fad54e66ce 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -1436,7 +1436,8 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *given_globals,
         PyObject *msg = PyUnicode_FromFormat("import of %R halted; "
                                              "None in sys.modules", abs_name);
         if (msg != NULL) {
-            PyErr_SetImportError(msg, abs_name, NULL);
+            PyErr_SetImportErrorSubclass(PyExc_ModuleNotFoundError, msg,
+                    abs_name, NULL);
             Py_DECREF(msg);
         }
         mod = NULL;
diff --git a/Python/importlib.h b/Python/importlib.h
index 406763103d..e2f60c905d 100644
--- a/Python/importlib.h
+++ b/Python/importlib.h
@@ -3239,8 +3239,8 @@ const unsigned char _Py_M__importlib[] = {
     0,114,64,1,0,0,114,59,1,0,0,114,96,0,0,0,
     114,56,1,0,0,114,7,0,0,0,114,145,0,0,0,114,
     69,1,0,0,244,11,0,0,0,95,103,99,100,95,105,109,
-    112,111,114,116,114,97,0,0,0,114,46,0,0,0,114,156,
-    0,0,0,114,98,0,0,0,40,5,0,0,0,114,66,0,
+    112,111,114,116,114,97,0,0,0,114,46,0,0,0,114,66,
+    1,0,0,114,98,0,0,0,40,5,0,0,0,114,66,0,
     0,0,114,57,1,0,0,114,58,1,0,0,114,160,0,0,
     0,114,139,0,0,0,114,4,0,0,0,114,4,0,0,0,
     114,5,0,0,0,114,70,1,0,0,61,6,0,0,115,26,