Index: Python/import.c
===================================================================
--- Python/import.c	(Revision 46240)
+++ Python/import.c	(Arbeitskopie)
@@ -1240,7 +1240,33 @@
 			if (importer == NULL)
 				return NULL;
 			/* Note: importer is a borrowed reference */
-			if (importer != Py_None) {
+			if (importer == Py_False) {
+				/* Cached as not being a valid dir. */
+				Py_XDECREF(copy);
+				continue;
+			}
+			else if (importer == Py_True) {
+				/* Cached as being a valid dir, so just
+				 * continue below. */
+			}
+			else if (importer == Py_None) {
+				/* No importer was found, so it has to be a file.
+				 * Check if the directory is valid. */
+#ifdef HAVE_STAT
+				if (stat(buf, &statbuf) != 0) {
+					/* Directory does not exist. */
+					PyDict_SetItem(path_importer_cache,
+					               v, Py_False);
+					Py_XDECREF(copy);
+					continue;
+				} else {
+					PyDict_SetItem(path_importer_cache,
+					               v, Py_True);
+				}
+#endif
+			}
+			else {
+				/* A real import hook importer was found. */
 				PyObject *loader;
 				loader = PyObject_CallMethod(importer,
 							     "find_module",
@@ -1253,9 +1279,11 @@
 					return &importhookdescr;
 				}
 				Py_DECREF(loader);
+				Py_XDECREF(copy);
+				continue;
 			}
-			/* no hook was successful, use builtin import */
 		}
+		/* no hook was found, use builtin import */
 
 		if (len > 0 && buf[len-1] != SEP
 #ifdef ALTSEP