Handle importing pkg.mod by executing

__import__('mod', {'__packaging__': 'pkg', level=1) w/o properly (and
thus not segfaulting).

Lib/importlib/_bootstrap.py

@@ -1083,7 +1083,7 @@ def __import__(name, globals={}, locals={}, fromlist=[], level=0):
             return module
         else:
             cut_off = len(name) - len(name.partition('.')[0])
-            return sys.modules[module.__name__[:-cut_off]]
+            return sys.modules[module.__name__[:len(module.__name__)-cut_off]]
     else:
         return _handle_fromlist(module, fromlist, _gcd_import)
 

Lib/importlib/test/import_/test_relative_imports.py

@@ -193,6 +193,15 @@ class RelativeImports(unittest.TestCase):
             self.assertEqual(module.__name__, '__runpy_pkg__.uncle.cousin')
         self.relative_import_test(create, globals_, callback)
 
+    def test_import_relative_import_no_fromlist(self):
+        # Import a relative module w/ no fromlist.
+        create = ['crash.__init__', 'crash.mod']
+        globals_ = [{'__package__': 'crash', '__name__': 'crash'}]
+        def callback(global_):
+            import_util.import_('crash')
+            mod = import_util.import_('mod', global_, {}, [], 1)
+            self.assertEqual(mod.__name__, 'crash.mod')
+        self.relative_import_test(create, globals_, callback)
 
 
 def test_main():

Python/import.c

@@ -3016,20 +3016,33 @@ PyImport_ImportModuleLevelObject(PyObject *name, PyObject *given_globals,
             Py_DECREF(partition);
 
             if (level == 0) {
-                final_mod = PyDict_GetItemWithError(interp->modules, front);
+                final_mod = PyDict_GetItem(interp->modules, front);
                 Py_DECREF(front);
-                Py_XINCREF(final_mod);
+                if (final_mod == NULL) {
+                    PyErr_Format(PyExc_KeyError,
+                                 "%R not in sys.modules as expected", front);
+                }
+                else {
+                    Py_INCREF(final_mod);
+                }
             }
             else {
                 Py_ssize_t cut_off = PyUnicode_GetLength(name) -
                                         PyUnicode_GetLength(front);
                 Py_ssize_t abs_name_len = PyUnicode_GetLength(abs_name);
-                PyObject *to_return = PyUnicode_Substring(name, 0,
+                PyObject *to_return = PyUnicode_Substring(abs_name, 0,
                                                         abs_name_len - cut_off);
 
                 final_mod = PyDict_GetItem(interp->modules, to_return);
-                Py_INCREF(final_mod);
                 Py_DECREF(to_return);
+                if (final_mod == NULL) {
+                    PyErr_Format(PyExc_KeyError,
+                                 "%R not in sys.modules as expected",
+                                 to_return);
+                }
+                else {
+                    Py_INCREF(final_mod);
+                }
             }
         }
         else {