Avoid None dummy value in node.RemoteNode.network.

Use a _DummyNetwork object instead.  Fix up check for "network is
None" by providing a convenience has_network() function in the base
class.

canopen/node/base.py

@@ -26,3 +26,7 @@ def __init__(
         self.object_dictionary = object_dictionary
 
         self.id = node_id or self.object_dictionary.node_id
+
+    def has_network(self) -> bool:
+        """Check whether the node has been associated to a network."""
+        return not isinstance(self.network, canopen.network._DummyNetwork)

canopen/node/remote.py

@@ -1,6 +1,9 @@
+from __future__ import annotations
+
 import logging
 from typing import Union, TextIO
 
+import canopen.network
 from canopen.sdo import SdoClient, SdoCommunicationError, SdoAbortedError
 from canopen.nmt import NmtMaster
 from canopen.emcy import EmcyConsumer
@@ -46,7 +49,7 @@ def __init__(
         if load_od:
             self.load_configuration()
 
-    def associate_network(self, network):
+    def associate_network(self, network: canopen.network.Network):
         self.network = network
         self.sdo.network = network
         self.pdo.network = network
@@ -59,18 +62,18 @@ def associate_network(self, network):
         network.subscribe(0x80 + self.id, self.emcy.on_emcy)
         network.subscribe(0, self.nmt.on_command)
 
-    def remove_network(self):
+    def remove_network(self) -> None:
         for sdo in self.sdo_channels:
             self.network.unsubscribe(sdo.tx_cobid, sdo.on_response)
         self.network.unsubscribe(0x700 + self.id, self.nmt.on_heartbeat)
         self.network.unsubscribe(0x80 + self.id, self.emcy.on_emcy)
         self.network.unsubscribe(0, self.nmt.on_command)
-        self.network = None
-        self.sdo.network = None
-        self.pdo.network = None
-        self.tpdo.network = None
-        self.rpdo.network = None
-        self.nmt.network = None
+        self.network = canopen.network._DummyNetwork()
+        self.sdo.network = self.network
+        self.pdo.network = self.network
+        self.tpdo.network = self.network
+        self.rpdo.network = self.network
+        self.nmt.network = self.network
 
     def add_sdo(self, rx_cobid, tx_cobid):
         """Add an additional SDO channel.
@@ -87,7 +90,7 @@ def add_sdo(self, rx_cobid, tx_cobid):
         """
         client = SdoClient(rx_cobid, tx_cobid, self.object_dictionary)
         self.sdo_channels.append(client)
-        if self.network is not None:
+        if self.has_network():
             self.network.subscribe(client.tx_cobid, client.on_response)
         return client