avoid jsonlib returning Decimal

The only functional change is that Decimal objects will no longer come out of jsonlib, but the code that handles jsonlib's two minor differences wrt other json libraries is now a bit clearer and better documented.

Relevant test is added.

IPython/zmq/session.py

@@ -73,9 +73,22 b' def squash_unicode(obj):'
 # globals and defaults
 #-----------------------------------------------------------------------------
 
-key = 'on_unknown' if jsonapi.jsonmod.__name__ == 'jsonlib' else 'default'
-json_packer = lambda obj: jsonapi.dumps(obj, **{key:date_default})
-json_unpacker = lambda s: extract_dates(jsonapi.loads(s))
+
+# jsonlib behaves a bit differently, so handle that where it affects us
+if jsonapi.jsonmod.__name__ == 'jsonlib':
+    # kwarg for serializing unknown types (datetime) is different
+    dumps_kwargs = dict(on_unknown=date_default)
+    # By default, jsonlib unpacks floats as Decimal instead of float,
+    # which can foul things up
+    loads_kwargs = dict(use_float=True)
+else:
+    # ISO8601-ify datetime objects
+    dumps_kwargs = dict(default=date_default)
+    # nothing to specify for loads
+    loads_kwargs = dict()
+
+json_packer = lambda obj: jsonapi.dumps(obj, **dumps_kwargs)
+json_unpacker = lambda s: extract_dates(jsonapi.loads(s, **loads_kwargs))
 
 pickle_packer = lambda o: pickle.dumps(o,-1)
 pickle_unpacker = pickle.loads

IPython/zmq/tests/test_session.py

@@ -59,7 +59,7 b' class TestSession(SessionTestCase):'
         self.assertEquals(msg['msg_type'], 'execute')
 
     def test_serialize(self):
-        msg = self.session.msg('execute',content=dict(a=10))
+        msg = self.session.msg('execute', content=dict(a=10, b=1.1))
         msg_list = self.session.serialize(msg, ident=b'foo')
         ident, msg_list = self.session.feed_identities(msg_list)
         new_msg = self.session.unserialize(msg_list)
@@ -69,6 +69,8 b' class TestSession(SessionTestCase):'
         self.assertEquals(new_msg['header'],msg['header'])
         self.assertEquals(new_msg['content'],msg['content'])
         self.assertEquals(new_msg['parent_header'],msg['parent_header'])
+        # ensure floats don't come out as Decimal:
+        self.assertEquals(type(new_msg['content']['b']),type(new_msg['content']['b']))
 
     def test_send(self):
         socket = MockSocket(zmq.Context.instance(),zmq.PAIR)