Separate subscription from the PDO's read() and save() methods.

Normally one needs to call read() or save() on a single pdo (or the
node's whole PDO collection) in order to receive any such objects from
the network.  That however requires quite a few SDO exchanges to make
sure the node's PDO configuration matches the parameters configured in
the object.

For applications where the PDO configuration is stored persistently in
the node (e.g. device EEPROM), doing this SDO exchange can be skipped
entirely if the application programmer takes care to mirror the same
configuration in the python-canopen objects.  Another use case is
reconnecting to a node for which the same python-canopen script
previously ran and the PDO configuration is still known to be valid.

Factor out a new method subscribe() from read() and save() to offer
doing only that last part via the public API.  Adapt the log message
and make sure it is logged in read() as well.

Author: acolomb

canopen/pdo/base.py

@@ -59,6 +59,17 @@ def save(self):
         for pdo_map in self.map.values():
             pdo_map.save()
 
+    def subscribe(self):
+        """Register the node's PDOs for reception on the network.
+
+        This normally happens when the PDO configuration is read from
+        or saved to the node.  Use this method to avoid the SDO flood
+        associated with read() or save(), if the local PDO setup is
+        known to match what's stored on the node.
+        """
+        for pdo_map in self.map.values():
+            pdo_map.subscribe()
+
     def export(self, filename):
         """Export current configuration to a database file.
 
@@ -331,8 +342,7 @@ def read(self):
             if index and size:
                 self.add_variable(index, subindex, size)
 
-        if self.enabled:
-            self.pdo_node.network.subscribe(self.cob_id, self.on_message)
+        self.subscribe()
 
     def save(self):
         """Save PDO configuration for this map using SDO."""
@@ -387,9 +397,19 @@ def save(self):
             self._update_data_size()
 
         if self.enabled:
-            logger.info("Enabling PDO")
             self.com_record[1].raw = self.cob_id | (RTR_NOT_ALLOWED if not self.rtr_allowed else 0x0)
+            self.subscribe()
+
+    def subscribe(self):
+        """Register the PDO for reception on the network.
 
+        This normally happens when the PDO configuration is read from
+        or saved to the node.  Use this method to avoid the SDO flood
+        associated with read() or save(), if the local PDO setup is
+        known to match what's stored on the node.
+        """
+        if self.enabled:
+            logger.info("Subscribing to enabled PDO 0x%X on the network", self.cob_id)
             self.pdo_node.network.subscribe(self.cob_id, self.on_message)
 
     def clear(self):