Issue #15778: Coerce ImportError.args to a string when it isn't

already one.

Patch by Dave Malcolm.

Lib/test/test_exceptions.py

@@ -937,6 +937,11 @@ class ImportErrorTests(unittest.TestCase):
         self.assertEqual(exc.name, 'somename')
         self.assertEqual(exc.path, 'somepath')
 
+    def test_non_str_argument(self):
+        # Issue #15778
+        arg = b'abc'
+        exc = ImportError(arg)
+        self.assertEqual(str(arg), str(exc))
 
 
 def test_main():

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 3.3.0 Release Candidate 1?
 Core and Builtins
 -----------------
 
+- Issue #15778: ensure that str(ImportError(msg)) returns a str
+  even when msg isn't a str.
+
 - Issue #2051: Source file permission bits are once again correctly
   copied to the cached bytecode file. (The migration to importlib
   reintroduced this problem because these was no regression test. A test

Objects/exceptions.c

@@ -679,7 +679,7 @@ ImportError_traverse(PyImportErrorObject *self, visitproc visit, void *arg)
 static PyObject *
 ImportError_str(PyImportErrorObject *self)
 {
-    if (self->msg) {
+    if (self->msg && PyUnicode_CheckExact(self->msg)) {
         Py_INCREF(self->msg);
         return self->msg;
     }