Allow mutable values in Constant trait, add tests

traits/tests/test_constant.py

@@ -0,0 +1,54 @@
+# (C) Copyright 2005-2020 Enthought, Inc., Austin, TX
+# All rights reserved.
+#
+# This software is provided without warranty under the terms of the BSD
+# license included in LICENSE.txt and may be redistributed only under
+# the conditions described in the aforementioned license. The license
+# is also available online at http://www.enthought.com/licenses/BSD.txt
+#
+# Thanks for using Enthought open source!
+
+
+import unittest
+
+from traits.api import Constant, HasTraits, TraitError
+
+
+class TestConstantTrait(unittest.TestCase):
+
+    def test_initial_value(self):
+        class TestClass(HasTraits):
+            c_atr = Constant(5)
+
+        self.assertEqual(5, TestClass().c_atr)
+
+    def test_mutable_initial_value(self):
+        class TestClass(HasTraits):
+            c_atr_1 = Constant([1, 2, 3, 4, 5])
+            c_atr_2 = Constant({"a": 1, "b": 2})
+
+        obj = TestClass()
+
+        self.assertEqual([1, 2, 3, 4, 5], obj.c_atr_1)
+        self.assertEqual({"a": 1, "b": 2}, obj.c_atr_2)
+
+    def test_assign_fails(self):
+        class TestClass(HasTraits):
+            c_atr = Constant(5)
+
+        with self.assertRaises(TraitError):
+            TestClass(c_atr=5)
+        with self.assertRaises(TraitError):
+            del TestClass().c_atr
+
+    def test_mutate_succeeds(self):
+        class TestClass(HasTraits):
+            c_atr_1 = Constant([1, 2, 3, 4, 5])
+            c_atr_2 = Constant({"a": 1, "b": 2})
+
+        obj = TestClass()
+        obj.c_atr_1.append(6)
+        obj.c_atr_2["c"] = 3
+
+        self.assertEqual([1, 2, 3, 4, 5, 6], obj.c_atr_1)
+        self.assertEqual({"a": 1, "b": 2, "c": 3}, obj.c_atr_2)

traits/trait_types.py

@@ -69,7 +69,6 @@
 
 # Constants
 
-MutableTypes = (list, dict)
 SetTypes = SequenceTypes + (set,)
 
 # Numeric type fast validator definitions
@@ -1029,14 +1028,6 @@ class Constant(TraitType):
     #: The standard metadata for the trait:
     metadata = {"type": "constant", "transient": True}
 
-    def __init__(self, value, **metadata):
-        if type(value) in MutableTypes:
-            raise TraitError(
-                "Cannot define a constant using a mutable list or dictionary"
-            )
-
-        super(Constant, self).__init__(value, **metadata)
-
 
 class Delegate(TraitType):
     """ A trait type whose value is delegated to a trait on another object.