give exception a nice message (closes #22379)

Patch by Yongzhi Pan.

6 files changed, 20 insertions, 22 deletions

diff --git a/Lib/test/string_tests.py b/Lib/test/string_tests.py
index 0479601..7818112 100644
--- a/Lib/test/string_tests.py
+++ b/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:
diff --git a/Lib/test/test_string.py b/Lib/test/test_string.py
index 0d07b91..f69255b 100644
--- a/Lib/test/test_string.py
+++ b/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)
diff --git a/Lib/test/test_userstring.py b/Lib/test/test_userstring.py
index 51d8e8b..9cca14a 100644
--- a/Lib/test/test_userstring.py
+++ b/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)
diff --git a/Misc/ACKS b/Misc/ACKS
index 3fb951c..fad0d88 100644
--- a/Misc/ACKS
+++ b/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
diff --git a/Misc/NEWS b/Misc/NEWS
index da2c554..237da41 100644
--- a/Misc/NEWS
+++ b/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.
 
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index 0b6d36c..f95857a 100644
--- a/Objects/stringobject.c
+++ b/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;
     }