Raise ValueError for unknown formats in the import/export OD APIs (#476)

Fixes #475

* Test the offending suffix is part of the error message
* Make the exception message more helpful

Author: erlend-aasland

canopen/objectdictionary/__init__.py

@@ -50,7 +50,11 @@ def export_od(od, dest: Union[str, TextIO, None] = None, doc_type: Optional[str]
             from canopen.objectdictionary import eds
             return eds.export_dcf(od, dest)
         else:
-            raise NotImplementedError(f"No support for the {doc_type!r} format")
+            allowed = ", ".join(doctypes)
+            raise ValueError(
+                f"Cannot export to the {doc_type!r} format; "
+                f"supported formats: {allowed}"
+            )
     finally:
         # If dest is opened in this fn, it should be closed
         if opened_here:
@@ -88,7 +92,12 @@ def import_od(
         from canopen.objectdictionary import epf
         return epf.import_epf(source)
     else:
-        raise NotImplementedError("No support for this format")
+        doc_type = suffix[1:]
+        allowed = ", ".join(["eds", "dcf", "epf"])
+        raise ValueError(
+            f"Cannot import from the {doc_type!r} format; "
+            f"supported formats: {allowed}"
+        )
 
 
 class ObjectDictionary(MutableMapping):

test/test_eds.py

@@ -54,6 +54,10 @@ def test_load_nonexisting_file(self):
         with self.assertRaises(IOError):
             canopen.import_od('/path/to/wrong_file.eds')
 
+    def test_load_unsupported_format(self):
+        with self.assertRaisesRegex(ValueError, "'py'"):
+            canopen.import_od(__file__)
+
     def test_load_file_object(self):
         with open(EDS_PATH) as fp:
             od = canopen.import_od(fp)
@@ -198,7 +202,7 @@ def test_export_eds(self):
                 self.verify_od(tempfile, doctype)
 
             # Test for unknown doctype
-            with self.assertRaises(NotImplementedError):
+            with self.assertRaisesRegex(ValueError, "'unknown'"):
                 tempfile = str(Path(tempdir, "test.unknown"))
                 canopen.export_od(self.od, tempfile, doc_type="unknown")