Fix: raise ValueError when manipulating an empty form

bddrest/authoring/given.py

@@ -46,6 +46,9 @@ def when(self, *args, record=True, **kwargs):
         # Checking for dictionary manipulators if any
         for k, v in kwargs.items():
             if isinstance(v, Manipulator):
+                basevalue = getattr(self.base_call, k)
+                if basevalue is None:
+                    raise ValueError(f'{k} argument is not given yet')
                 clone = getattr(self.base_call, k).copy()
                 v.apply(clone)
                 kwargs[k] = clone

tests/test_form_manipulation.py

@@ -44,6 +44,16 @@ def test_append_form_field():
             email='user@example.com'
         )
 
+    call = dict(
+        title='test add form field',
+        url='/apiv1/devices/name: SM-12345678/id: 1',
+        verb='POST',
+    )
+    with Given(wsgi_application, **call):
+        with pytest.raises(ValueError):
+            when('Appending a field to an empty form',
+                 form=Append('email', 'foo@bar.baz'))
+
 
 def test_remove_from_fields():
     call = dict(
@@ -63,6 +73,10 @@ def test_remove_from_fields():
         when('Removing fields', form=Remove('email', 'phone'))
         assert response.json == dict(activationCode='746727')
 
+        with pytest.raises(ValueError):
+            when('Removing a field from an empty form',
+                 json=Remove('email', 'phone'))
+
         with pytest.raises(ValueError):
             Remove('a').apply(['b', 'c'])