pdo: Fix wrongfully returned object for missing index (#636)

The fallback behavior in PdoMaps.__getitem__() tried looking up by
parameter record index even when no com_offset or map_offset was
defined. This caused a possibly wrong PdoMap object to be
returned. For example with 0x15FF, the lookup for 0x1600 would
succeed.

Add a test to make sure that an nonexistant parameter record index for
0x15FF actually does raise a KeyError. Skip the fallback lookup by
parameter index if no record offsets are defined.

Author: acolomb

canopen/pdo/base.py

@@ -175,10 +175,12 @@ def __getitem__(self, key: int) -> PdoMap:
         try:
             return self.maps[key]
         except KeyError:
-            with contextlib.suppress(KeyError):
-                return self.maps[key + 1 - self.map_offset]
-            with contextlib.suppress(KeyError):
-                return self.maps[key + 1 - self.com_offset]
+            if self.map_offset:
+                with contextlib.suppress(KeyError):
+                    return self.maps[key + 1 - self.map_offset]
+            if self.com_offset:
+                with contextlib.suppress(KeyError):
+                    return self.maps[key + 1 - self.com_offset]
             raise
 
     def __iter__(self) -> Iterator[int]:

test/test_pdo.py

@@ -80,6 +80,7 @@ def test_pdo_getitem(self):
         self.assertRaises(KeyError, lambda: node.pdo['DOES NOT EXIST'])
         self.assertRaises(KeyError, lambda: node.pdo[0x1BFF])
         self.assertRaises(KeyError, lambda: node.tpdo[0x1BFF])
+        self.assertRaises(KeyError, lambda: node.pdo[0x15FF])
 
     def test_pdo_iterate(self):
         node = self.node