Fix some type checker errors in network module (#534)

* Annotate PeriodicMessageTask in NmtMaster and NmtSlave.

* Type annotate Network.notifier.

* Remove duplicate initialization of PeriodicMessageTask._task.

The attribute is set within the method directly following.  So there
is no need to initialize it to None before, only to confuse type
checkers.

canopen/network.py

@@ -41,7 +41,7 @@ def __init__(self, bus: Optional[can.BusABC] = None):
         #: List of :class:`can.Listener` objects.
         #: Includes at least MessageListener.
         self.listeners = [MessageListener(self)]
-        self.notifier = None
+        self.notifier: Optional[can.Notifier] = None
         self.nodes: Dict[int, Union[RemoteNode, LocalNode]] = {}
         self.subscribers: Dict[int, List[Callback]] = {}
         self.send_lock = threading.Lock()
@@ -311,7 +311,6 @@ def __init__(
         self.msg = can.Message(is_extended_id=can_id > 0x7FF,
                                arbitration_id=can_id,
                                data=data, is_remote_frame=remote)
-        self._task = None
         self._start()
 
     def _start(self):

canopen/nmt.py

@@ -2,7 +2,11 @@
 import logging
 import struct
 import time
-from typing import Callable, Optional
+from typing import Callable, Optional, TYPE_CHECKING
+
+if TYPE_CHECKING:
+    from canopen.network import PeriodicMessageTask
+
 
 logger = logging.getLogger(__name__)
 
@@ -107,7 +111,7 @@ class NmtMaster(NmtBase):
     def __init__(self, node_id: int):
         super(NmtMaster, self).__init__(node_id)
         self._state_received = None
-        self._node_guarding_producer = None
+        self._node_guarding_producer: Optional[PeriodicMessageTask] = None
         #: Timestamp of last heartbeat message
         self.timestamp: Optional[float] = None
         self.state_update = threading.Condition()
@@ -197,7 +201,7 @@ class NmtSlave(NmtBase):
 
     def __init__(self, node_id: int, local_node):
         super(NmtSlave, self).__init__(node_id)
-        self._send_task = None
+        self._send_task: Optional[PeriodicMessageTask] = None
         self._heartbeat_time_ms = 0
         self._local_node = local_node