DS402: Minimize side-effects of operation mode switching (ref #244) (#251)

* ds402: Keep target values on operation mode change.

As the comment already stated, clearing the target values should
possibly happen before switching to the OPERATION ENABLED state, to
avoid unexpected movements.  So doing that when actually leaving that
state is mostly useless.

Some legitimate use cases even require switching the operation mode
while in OPERATION ENABLED, e.g. switching between Profile Position
and Profile Velocity.  This change does not allow that, but at the
very least will avoid the need to reset target values again.

* ds402: Keep power state on operation mode change.

Some legitimate use cases require switching the operation mode while
in OPERATION ENABLED, e.g. switching between Profile Position and
Profile Velocity.

If an application or specific controller needs the transition from
OPERATION ENABLED to SWITCHED ON during operation mode changes, that
should be handled outside this library, and is easy enough to do.  On
the other hand, having it inside the op_mode setter prevents the above
mentioned use-case.

* ds402: Improve logging in op_mode setter.

Do not generate a log message about the changed operation mode when it
actually failed.  Use a consistent style for the TypeError message
formatting.

Author: acolomb

canopen/profiles/p402.py

@@ -361,31 +361,20 @@ def op_mode(self, mode):
         try:
             if not self.is_op_mode_supported(mode):
                 raise TypeError(
-                    'Operation mode {0} not suppported on node {1}.'.format(mode, self.id))
-
-            start_state = self.state
-
-            if self.state == 'OPERATION ENABLED':
-                self.state = 'SWITCHED ON'
-                # ensure the node does not move with an old value
-                self._clear_target_values() # Shouldn't this happen before it's switched on?
-                
+                    'Operation mode {m} not suppported on node {n}.'.format(n=self.id, m=mode))
             # operation mode
             self.sdo[0x6060].raw = OperationMode.NAME2CODE[mode]
-
             timeout = time.monotonic() + self.TIMEOUT_SWITCH_OP_MODE
             while self.op_mode != mode:
                 if time.monotonic() > timeout:
                     raise RuntimeError(
                         "Timeout setting node {0}'s new mode of operation to {1}.".format(
                             self.id, mode))
+            logger.info('Set node {n} operation mode to {m}.'.format(n=self.id, m=mode))
         except SdoCommunicationError as e:
             logger.warning('[SDO communication error] Cause: {0}'.format(str(e)))
         except (RuntimeError, ValueError) as e:
             logger.warning('{0}'.format(str(e)))
-        finally:
-            self.state = start_state # why?
-            logger.info('Set node {n} operation mode to {m}.'.format(n=self.id, m=mode))
 
     def _clear_target_values(self):
         # [target velocity, target position, target torque]