Backport #1172 and #1173 to the release branch

CHANGES.rst

@@ -164,7 +164,7 @@ Features
 * Add ``Map`` and ``PrefixMap`` trait types. (#886, #953, #956, #970, #1139)
 * Add ``TraitList`` as the base list object that can perform validation
   and emit change notifications. (#912, #981, #984, #989, #999, #1003, #1011,
-  #1026, #1009, #1040)
+  #1026, #1009, #1040, #1172, #1173)
 * Add ``TraitDict`` as the base dict object that can perform validation and
   emit change notifications. (#913)
 * Add ``TraitSet`` as the base set object that can perform validation and

traits/tests/test_trait_list_object.py

@@ -1150,6 +1150,8 @@ class HasLengthConstrainedLists(HasTraits):
 
     at_most_five = List(Int, maxlen=5)
 
+    unconstrained = List(Int)
+
 
 class TestTraitListObject(unittest.TestCase):
     def test_list_of_lists_pickle_with_notifier(self):
@@ -1214,6 +1216,11 @@ def test_delitem_slice_too_small(self):
             del foo.at_least_two[:]
         self.assertEqual(foo.at_least_two, [1, 2])
 
+    def test_delitem_from_empty(self):
+        foo = HasLengthConstrainedLists()
+        with self.assertRaises(IndexError):
+            del foo.unconstrained[0]
+
     def test_iadd(self):
         foo = HasLengthConstrainedLists(at_most_five=[1, 2])
         foo.at_most_five += [6, 7, 8]
@@ -1381,6 +1388,13 @@ def test_pop_too_small(self):
             foo.at_least_two.pop(10)
         self.assertEqual(foo.at_least_two, [1, 2])
 
+    def test_pop_from_empty(self):
+        foo = HasLengthConstrainedLists()
+        with self.assertRaises(IndexError):
+            foo.unconstrained.pop()
+        with self.assertRaises(IndexError):
+            foo.unconstrained.pop(10)
+
     def test_remove(self):
         foo = HasLengthConstrainedLists(at_least_two=[1, 2, 6, 4])
         foo.at_least_two.remove(2)
@@ -1398,6 +1412,23 @@ def test_remove_too_small(self):
             foo.at_least_two.remove(10)
         self.assertEqual(foo.at_least_two, [1, 2])
 
+    def test_remove_from_empty(self):
+        foo = HasLengthConstrainedLists()
+        with self.assertRaises(ValueError):
+            foo.unconstrained.remove(35)
+
+    def test_length_violation_error_message(self):
+        # Regression test for enthought/traits#1170
+        foo = HasLengthConstrainedLists(at_least_two=[1, 2])
+        with self.assertRaises(TraitError) as exc_cm:
+            foo.at_least_two.remove(1)
+
+        exc_message = str(exc_cm.exception)
+        self.assertIn("'at_least_two' trait", exc_message)
+        self.assertIn("HasLengthConstrainedLists instance", exc_message)
+        self.assertIn("an integer", exc_message)
+        self.assertIn("at least 2 items", exc_message)
+
     def test_dead_object_reference(self):
         foo = HasLengthConstrainedLists(at_most_five=[1, 2, 3, 4])
         list_object = foo.at_most_five

traits/trait_list_object.py

@@ -634,7 +634,7 @@ def __delitem__(self, key):
         """
 
         removed_count = len(self[key]) if isinstance(key, slice) else 1
-        self._validate_length(len(self) - removed_count)
+        self._validate_length(max(len(self) - removed_count, 0))
         super().__delitem__(key)
 
     def __iadd__(self, value):
@@ -776,7 +776,7 @@ def pop(self, index=-1):
             If list is empty or index is out of range.
         """
 
-        self._validate_length(len(self) - 1)
+        self._validate_length(max(len(self) - 1, 0))
         return super().pop(index)
 
     def remove(self, value):
@@ -797,7 +797,7 @@ def remove(self, value):
             If the value is not present.
         """
 
-        self._validate_length(len(self) - 1)
+        self._validate_length(max(len(self) - 1, 0))
         super().remove(value)
 
     # -- pickle and copy support ----------------------------------------------
@@ -889,8 +889,8 @@ def _validate_length(self, new_length):
                 "but you attempted to change its length to %d %s."
                 % (
                     self.name,
-                    class_of(object),
-                    self.trait.full_info(object, self.name, Undefined),
+                    class_of(self.object()),
+                    self.trait.full_info(self.object(), self.name, Undefined),
                     new_length,
                     "element" if new_length == 1 else "elements",
                 )