give exception a nice message (closes #22379)

Patch by Yongzhi Pan.

Lib/test/string_tests.py

@@ -65,14 +65,12 @@ class CommonTest(unittest.TestCase):
             self.assertTrue(object is not realresult)
 
     # check that object.method(*args) raises exc
-    def checkraises(self, exc, object, methodname, *args):
-        object = self.fixtype(object)
+    def checkraises(self, exc, obj, methodname, *args):
+        obj = self.fixtype(obj)
         args = self.fixtype(args)
-        self.assertRaises(
-            exc,
-            getattr(object, methodname),
-            *args
-        )
+        with self.assertRaises(exc) as cm:
+            getattr(obj, methodname)(*args)
+        self.assertNotEqual(cm.exception.message, '')
 
     # call object.method(*args) without any checks
     def checkcall(self, object, methodname, *args):
@@ -1057,6 +1055,7 @@ class MixinStrUnicodeUserStringTest:
         self.checkequal('a b c', ' ', 'join', BadSeq2())
 
         self.checkraises(TypeError, ' ', 'join')
+        self.checkraises(TypeError, ' ', 'join', None)
         self.checkraises(TypeError, ' ', 'join', 7)
         self.checkraises(TypeError, ' ', 'join', Sequence([7, 'hello', 123L]))
         try:

Lib/test/test_string.py

@@ -16,13 +16,10 @@ class StringTest(
             realresult
         )
 
-    def checkraises(self, exc, object, methodname, *args):
-        self.assertRaises(
-            exc,
-            getattr(string, methodname),
-            object,
-            *args
-        )
+    def checkraises(self, exc, obj, methodname, *args):
+        with self.assertRaises(exc) as cm:
+            getattr(string, methodname)(obj, *args)
+        self.assertNotEqual(cm.exception.message, '')
 
     def checkcall(self, object, methodname, *args):
         getattr(string, methodname)(object, *args)

Lib/test/test_userstring.py

@@ -28,14 +28,12 @@ class UserStringTest(
             realresult
         )
 
-    def checkraises(self, exc, object, methodname, *args):
-        object = self.fixtype(object)
+    def checkraises(self, exc, obj, methodname, *args):
+        obj = self.fixtype(obj)
         # we don't fix the arguments, because UserString can't cope with it
-        self.assertRaises(
-            exc,
-            getattr(object, methodname),
-            *args
-        )
+        with self.assertRaises(exc) as cm:
+            getattr(obj, methodname)(*args)
+        self.assertNotEqual(cm.exception.message, '')
 
     def checkcall(self, object, methodname, *args):
         object = self.fixtype(object)

Misc/ACKS

@@ -1007,6 +1007,7 @@ Mike Pall
 Todd R. Palmer
 Juan David Ibáñez Palomar
 Jan Palus
+Yongzhi Pan
 Mathias Panzenböck
 M. Papillon
 Peter Parente

Misc/NEWS

@@ -10,6 +10,9 @@ What's New in Python 2.7.9?
 Core and Builtins
 -----------------
 
+- Issue #22379: Fix empty exception message in a TypeError raised in
+  ``str.join``.
+
 - Issue #22221: Now the source encoding declaration on the second line isn't
   effective if the first line contains anything except a comment.
 

Objects/stringobject.c

@@ -1594,7 +1594,7 @@ string_join(PyStringObject *self, PyObject *orig)
     Py_ssize_t i;
     PyObject *seq, *item;
 
-    seq = PySequence_Fast(orig, "");
+    seq = PySequence_Fast(orig, "can only join an iterable");
     if (seq == NULL) {
         return NULL;
     }