add Session to ConnectionFileMixin

it’s used in a few places, so it doesn’t make sense for subclasses to lack it

IPython/kernel/client.py

@@ -56,11 +56,6 b' class KernelClient(LoggingConfigurable, ConnectionFileMixin):'
     def _context_default(self):
         return zmq.Context.instance()
 
-    # The Session to use for communication with the kernel.
-    session = Instance(Session)
-    def _session_default(self):
-        return Session(parent=self)
-
     # The classes to use for the various channels
     shell_channel_class = Type(ShellChannel)
     iopub_channel_class = Type(IOPubChannel)

IPython/kernel/connect.py

@@ -34,7 +34,7 b' from IPython.utils.path import filefind, get_ipython_dir'
 from IPython.utils.py3compat import (str_to_bytes, bytes_to_str, cast_bytes_py2,
                                      string_types)
 from IPython.utils.traitlets import (
-    Bool, Integer, Unicode, CaselessStrEnum,
+    Bool, Integer, Unicode, CaselessStrEnum, Instance,
 )
 
 
@@ -433,6 +433,12 b' class ConnectionFileMixin(Configurable):'
     def ports(self):
         return [ getattr(self, name) for name in port_names ]
 
+    # The Session to use for communication with the kernel.
+    session = Instance('IPython.kernel.zmq.session.Session')
+    def _session_default(self):
+        from IPython.kernel.zmq.session import Session
+        return Session(parent=self)
+
     #--------------------------------------------------------------------------
     # Connection and ipc file management
     #--------------------------------------------------------------------------
@@ -506,15 +512,10 b' class ConnectionFileMixin(Configurable):'
                 # not overridden by config or cl_args
                 setattr(self, name, cfg[name])
         
-        if self.session:
-            session = self.session
-        else:
-            session = self.config.Session
-        
         if 'key' in cfg:
-            session.key = str_to_bytes(cfg['key'])
+            self.session.key = str_to_bytes(cfg['key'])
         if 'signature_scheme' in cfg:
-            session.signature_scheme = cfg['signature_scheme']
+            self.session.signature_scheme = cfg['signature_scheme']
 
     #--------------------------------------------------------------------------
     # Creating connected sockets

IPython/kernel/manager.py

@@ -46,11 +46,6 b' class KernelManager(LoggingConfigurable, ConnectionFileMixin):'
     def _context_default(self):
         return zmq.Context.instance()
 
-    # The Session to use for communication with the kernel.
-    session = Instance(Session)
-    def _session_default(self):
-        return Session(parent=self)
-
     # the class to create with our `client` method
     client_class = DottedObjectName('IPython.kernel.blocking.BlockingKernelClient')
     client_factory = Type()

IPython/kernel/zmq/kernelapp.py

@@ -111,7 +111,6 b' class IPKernelApp(BaseIPythonApplication, InteractiveShellApp,'
     kernel = Any()
     poller = Any() # don't restrict this even though current pollers are all Threads
     heartbeat = Instance(Heartbeat)
-    session = Instance('IPython.kernel.zmq.session.Session')
     ports = Dict()
     
     # ipkernel doesn't get its own config file
@@ -289,11 +288,6 b' class IPKernelApp(BaseIPythonApplication, InteractiveShellApp,'
                                 stdin=self.stdin_port, hb=self.hb_port,
                                 control=self.control_port)
 
-    def init_session(self):
-        """create our session object"""
-        default_secure(self.config)
-        self.session = Session(parent=self, username=u'kernel')
-
     def init_blackhole(self):
         """redirects stdout/stderr to devnull if necessary"""
         if self.no_stdout or self.no_stderr:
@@ -363,9 +357,9 b' class IPKernelApp(BaseIPythonApplication, InteractiveShellApp,'
     @catch_config_error
     def initialize(self, argv=None):
         super(IPKernelApp, self).initialize(argv)
+        default_secure(self.config)
         self.init_blackhole()
         self.init_connection_file()
-        self.init_session()
         self.init_poller()
         self.init_sockets()
         self.init_heartbeat()