fixup! Add MapFields transformation

bonobo/nodes/basics.py

@@ -334,13 +334,19 @@ def MapFields(function, key=True):
     def _MapFields(bag):
         try:
             factory = type(bag)._make
-        except AttributeError as e:
-            raise UnrecoverableAttributeError('This transformation works only on objects with named'
-                                              ' fields (namedtuple, BagType, ...).') from e
+        except AttributeError:
+            factory = type(bag)
 
         if callable(key):
+            try:
+                fields = bag._fields
+            except AttributeError as e:
+                raise UnrecoverableAttributeError(
+                    'This transformation works only on objects with named'
+                    ' fields (namedtuple, BagType, ...).') from e
+
             return factory(
-                function(value) if key(key_) else value for key_, value in zip(bag._fields, bag)
+                function(value) if key(key_) else value for key_, value in zip(fields, bag)
             )
         elif key:
             return factory(function(value) for value in bag)

tests/nodes/test_basics.py

@@ -123,6 +123,8 @@ MyBag = BagType("MyBag", ["a", "b", "c"])
     (MyBag(1, 2, 3), True, MyBag(1, 4, 9)),
     (MyBag(1, 2, 3), False, MyBag(1, 2, 3)),
     (MyBag(1, 2, 3), lambda x: x == 'c', MyBag(1, 2, 9)),
+    ((1, 2, 3), True, (1, 4, 9)),
+    ((1, 2, 3), False, (1, 2, 3)),
 ])
 def test_map_fields(input_, key, expected):
     with BufferingNodeExecutionContext(bonobo.MapFields(lambda x: x**2, key)) as context:
@@ -133,7 +135,7 @@ def test_map_fields(input_, key, expected):
 
 
 def test_map_fields_error():
-    with BufferingNodeExecutionContext(bonobo.MapFields(lambda x: x**2)) as context:
+    with BufferingNodeExecutionContext(bonobo.MapFields(lambda x: x**2, lambda x: x == 'c')) as context:
         context.write_sync(tuple())
     assert context.status == '!'
     assert context.defunct