Merge pull request #4239 from minrk/better-errors-for-aaron

more informative error message for bad serialization

IPython/kernel/zmq/session.py

@@ -436,8 +436,15 b' class Session(Configurable):'
         msg = dict(a=[1,'hi'])
         try:
             packed = pack(msg)
-        except Exception:
-            raise ValueError("packer could not serialize a simple message")
+        except Exception as e:
+            msg = "packer '{packer}' could not serialize a simple message: {e}{jsonmsg}"
+            if self.packer == 'json':
+                jsonmsg = "\nzmq.utils.jsonapi.jsonmod = %s" % jsonapi.jsonmod
+            else:
+                jsonmsg = ""
+            raise ValueError(
+                msg.format(packer=self.packer, e=e, jsonmsg=jsonmsg)
+            )
 
         # ensure packed message is bytes
         if not isinstance(packed, bytes):
@@ -446,8 +453,16 b' class Session(Configurable):'
         # check that unpack is pack's inverse
         try:
             unpacked = unpack(packed)
-        except Exception:
-            raise ValueError("unpacker could not handle the packer's output")
+            assert unpacked == msg
+        except Exception as e:
+            msg = "unpacker '{unpacker}' could not handle output from packer '{packer}': {e}{jsonmsg}"
+            if self.packer == 'json':
+                jsonmsg = "\nzmq.utils.jsonapi.jsonmod = %s" % jsonapi.jsonmod
+            else:
+                jsonmsg = ""
+            raise ValueError(
+                msg.format(packer=self.packer, unpacker=self.unpacker, e=e, jsonmsg=jsonmsg)
+            )
 
         # check datetime support
         msg = dict(t=datetime.now())

IPython/kernel/zmq/tests/test_session.py

@@ -20,6 +20,12 b' from zmq.eventloop.zmqstream import ZMQStream'
 
 from IPython.kernel.zmq import session as ss
 
+def _bad_packer(obj):
+    raise TypeError("I don't work")
+
+def _bad_unpacker(bytes):
+    raise TypeError("I don't work either")
+
 class SessionTestCase(BaseZMQTestCase):
 
     def setUp(self):
@@ -222,4 +228,44 b' class TestSession(SessionTestCase):'
         self.assertTrue(len(session.digest_history) == 100)
         session._add_digest(uuid.uuid4().bytes)
         self.assertTrue(len(session.digest_history) == 91)
-
+    
+    def test_bad_pack(self):
+        try:
+            session = ss.Session(pack=_bad_packer)
+        except ValueError as e:
+            self.assertIn("could not serialize", str(e))
+            self.assertIn("don't work", str(e))
+        else:
+            self.fail("Should have raised ValueError")
+    
+    def test_bad_unpack(self):
+        try:
+            session = ss.Session(unpack=_bad_unpacker)
+        except ValueError as e:
+            self.assertIn("could not handle output", str(e))
+            self.assertIn("don't work either", str(e))
+        else:
+            self.fail("Should have raised ValueError")
+    
+    def test_bad_packer(self):
+        try:
+            session = ss.Session(packer=__name__ + '._bad_packer')
+        except ValueError as e:
+            self.assertIn("could not serialize", str(e))
+            self.assertIn("don't work", str(e))
+        else:
+            self.fail("Should have raised ValueError")
+    
+    def test_bad_unpacker(self):
+        try:
+            session = ss.Session(unpacker=__name__ + '._bad_unpacker')
+        except ValueError as e:
+            self.assertIn("could not handle output", str(e))
+            self.assertIn("don't work either", str(e))
+        else:
+            self.fail("Should have raised ValueError")
+    
+    def test_bad_roundtrip(self):
+        with self.assertRaises(ValueError):
+            session=  ss.Session(unpack=lambda b: 5)
+