allow setting Tuple traitlet with list

matches traits behavior

IPython/utils/tests/test_traitlets.py

@@ -886,8 +886,13 b' class TestList(TraitTestBase):'
     obj = ListTrait()
 
     _default_value = []
-    _good_values = [[], [1], list(range(10))]
-    _bad_values = [10, [1,'a'], 'a', (1,2)]
+    _good_values = [[], [1], list(range(10)), (1,2)]
+    _bad_values = [10, [1,'a'], 'a']
+    
+    def coerce(self, value):
+        if value is not None:
+            value = list(value)
+        return value
 
 class LenListTrait(HasTraits):
 
@@ -898,8 +903,13 b' class TestLenList(TraitTestBase):'
     obj = LenListTrait()
 
     _default_value = [0]
-    _good_values = [[1], list(range(2))]
-    _bad_values = [10, [1,'a'], 'a', (1,2), [], list(range(3))]
+    _good_values = [[1], [1,2], (1,2)]
+    _bad_values = [10, [1,'a'], 'a', [], list(range(3))]
+
+    def coerce(self, value):
+        if value is not None:
+            value = list(value)
+        return value
 
 class TupleTrait(HasTraits):
 
@@ -910,8 +920,13 b' class TestTupleTrait(TraitTestBase):'
     obj = TupleTrait()
 
     _default_value = None
-    _good_values = [(1,), None,(0,)]
-    _bad_values = [10, (1,2), [1],('a'), ()]
+    _good_values = [(1,), None, (0,), [1]]
+    _bad_values = [10, (1,2), ('a'), ()]
+
+    def coerce(self, value):
+        if value is not None:
+            value = tuple(value)
+        return value
 
     def test_invalid_args(self):
         self.assertRaises(TypeError, Tuple, 5)
@@ -927,8 +942,13 b' class TestLooseTupleTrait(TraitTestBase):'
     obj = LooseTupleTrait()
 
     _default_value = (1,2,3)
-    _good_values = [(1,), None, (0,), tuple(range(5)), tuple('hello'), ('a',5), ()]
-    _bad_values = [10, 'hello', [1], []]
+    _good_values = [(1,), None, [1], (0,), tuple(range(5)), tuple('hello'), ('a',5), ()]
+    _bad_values = [10, 'hello', {}]
+
+    def coerce(self, value):
+        if value is not None:
+            value = tuple(value)
+        return value
 
     def test_invalid_args(self):
         self.assertRaises(TypeError, Tuple, 5)

IPython/utils/traitlets.py

@@ -1210,6 +1210,7 b' class Container(Instance):'
     To be subclassed by overriding klass.
     """
     klass = None
+    _cast_types = ()
     _valid_defaults = SequenceTypes
     _trait = None
 
@@ -1273,6 +1274,8 b' class Container(Instance):'
         raise TraitError(e)
 
     def validate(self, obj, value):
+        if isinstance(value, self._cast_types):
+            value = self.klass(value)
         value = super(Container, self).validate(obj, value)
         if value is None:
             return value
@@ -1298,6 +1301,7 b' class Container(Instance):'
 class List(Container):
     """An instance of a Python list."""
     klass = list
+    _cast_types = (tuple,)
 
     def __init__(self, trait=None, default_value=None, minlen=0, maxlen=sys.maxsize,
                 allow_none=True, **metadata):
@@ -1354,15 +1358,27 b' class List(Container):'
             self.length_error(obj, value)
 
         return super(List, self).validate_elements(obj, value)
+    
+    def validate(self, obj, value):
+        value = super(List, self).validate(obj, value)
+        if value is None:
+            return value
+
+        value = self.validate_elements(obj, value)
+
+        return value
+        
 
 
-class Set(Container):
+class Set(List):
     """An instance of a Python set."""
     klass = set
+    _cast_types = (tuple, list)
 
 class Tuple(Container):
     """An instance of a Python tuple."""
     klass = tuple
+    _cast_types = (list,)
 
     def __init__(self, *traits, **metadata):
         """Tuple(*traits, default_value=None, allow_none=True, **medatata)