smell: fixes assert in container constructor.

bonobo/config/services.py

@@ -66,8 +66,11 @@ class Service(Option):
 class Container(dict):
     def __new__(cls, *args, **kwargs):
         if len(args) == 1:
-            assert not len(kwargs), 'only one usage at a time, my dear.'
-            if not (args[0]):
+            if len(kwargs):
+                raise ValueError(
+                    'You can either use {} with one positional argument or with keyword arguments, not both.'
+                )
+            if not args[0]:
                 return super().__new__(cls)
             if isinstance(args[0], cls):
                 return cls

tests/config/test_services.py

@@ -130,6 +130,20 @@ def test_requires():
     assert svcargs['output'] == vcr.append
 
 
+def test_constructor():
+    c1 = Container(foo='foo', bar='bar')
+    assert 2 == len(c1)
+
+    c2 = Container({'foo': 'foo', 'bar': 'bar'})
+    assert 2 == len(c2)
+
+    assert c1['foo'] == c2['foo']
+    assert c1['bar'] == c2['bar']
+
+    with pytest.raises(ValueError):
+        Container({'bar': 'bar'}, foo='foo')
+
+
 @pytest.mark.parametrize('services', [None, {}])
 def test_create_container_empty_values(services):
     c = create_container(services)