make deprecated shell_streams writable

ipykernel/ipkernel.py

@@ -162,7 +162,10 @@ def banner(self):
 
     def start(self):
         self.shell.exit_now = False
-        self.debugpy_stream.on_recv(self.dispatch_debugpy, copy=False)
+        if self.debugpy_stream is None:
+            self.log.warning("debugpy_stream undefined, debugging will not be enabled")
+        else:
+            self.debugpy_stream.on_recv(self.dispatch_debugpy, copy=False)
         super(IPythonKernel, self).start()
 
     def set_parent(self, ident, parent, channel='shell'):

ipykernel/kernelbase.py

@@ -59,13 +59,41 @@ def _update_eventloop(self, change):
     profile_dir = Instance('IPython.core.profiledir.ProfileDir', allow_none=True)
     shell_stream = Instance(ZMQStream, allow_none=True)
 
-    @property
-    def shell_streams(self):
+    shell_streams = List(
+        help="""Deprecated shell_streams alias. Use shell_stream
+
+        .. versionchanged:: 6.0
+            shell_streams is deprecated. Use shell_stream.
+        """
+    )
+
+    @default("shell_streams")
+    def _shell_streams_default(self):
         warnings.warn(
-            'Property shell_streams is deprecated in favor of shell_stream',
-            DeprecationWarning
+            "Kernel.shell_streams is deprecated in ipykernel 6.0. Use Kernel.shell_stream",
+            DeprecationWarning,
+            stacklevel=2,
         )
-        return [self.shell_stream]
+        if self.shell_stream is not None:
+            return [self.shell_stream]
+        else:
+            return []
+
+    @observe("shell_streams")
+    def _shell_streams_changed(self, change):
+        warnings.warn(
+            "Kernel.shell_streams is deprecated in ipykernel 6.0. Use Kernel.shell_stream",
+            DeprecationWarning,
+            stacklevel=2,
+        )
+        if len(change.new) > 1:
+            warnings.warn(
+                "Kernel only supports one shell stream. Additional streams will be ignored.",
+                RuntimeWarning,
+                stacklevel=2,
+            )
+        if change.new:
+            self.shell_stream = change.new[0]
 
     control_stream = Instance(ZMQStream, allow_none=True)
 
@@ -177,8 +205,6 @@ def __init__(self, **kwargs):
             self.control_handlers[msg_type] = getattr(self, msg_type)
 
         self.control_queue = Queue()
-        if 'control_thread' in kwargs:
-            kwargs['control_thread'].io_loop.add_callback(self.poll_control_queue)
 
     @gen.coroutine
     def dispatch_control(self, msg):
@@ -424,6 +450,13 @@ def start(self):
 
         self.control_stream.on_recv(self.dispatch_control, copy=False)
 
+        if self.control_thread:
+            control_loop = self.control_thread.io_loop
+        else:
+            control_loop = self.io_loop
+
+        control_loop.add_callback(self.poll_control_queue)
+
         self.shell_stream.on_recv(
             partial(
                 self.schedule_dispatch,