Add __slots__ to ObserverExpression and its subclasses

traits/observation/expression.py

@@ -32,6 +32,8 @@ class ObserverExpression:
     functions provided in this module, e.g. ``trait``.
     """
 
+    __slots__ = ()
+
     def __eq__(self, other):
         """ Return true if the other value is an ObserverExpression with
         equivalent content.
@@ -278,6 +280,8 @@ class SingleObserverExpression(ObserverExpression):
     """ Container of ObserverExpression for wrapping a single observer.
     """
 
+    __slots__ = ("observer",)
+
     def __init__(self, observer):
         self.observer = observer
 
@@ -298,6 +302,8 @@ class SeriesObserverExpression(ObserverExpression):
         Right expression to be joined in series.
     """
 
+    __slots__ = ("_first", "_second")
+
     def __init__(self, first, second):
         self._first = first
         self._second = second
@@ -318,6 +324,8 @@ class ParallelObserverExpression(ObserverExpression):
         Right expression to be joined in parallel.
     """
 
+    __slots__ = ("_left", "_right")
+
     def __init__(self, left, right):
         self._left = left
         self._right = right

traits/observation/tests/test_expression.py

@@ -683,6 +683,22 @@ def test_equality_different_type(self):
         self.assertNotEqual(expr, "1")
 
 
+class TestObserverExpressionSlots(unittest.TestCase):
+    """ Check that expressions use __slots__. """
+
+    def test_single_expression(self):
+        expr = create_expression(1)
+        self.assertFalse(hasattr(expr, "__dict__"))
+
+    def test_series_expression(self):
+        expr = create_expression(1).then(create_expression(2))
+        self.assertFalse(hasattr(expr, "__dict__"))
+
+    def test_parallel_expression(self):
+        expr = create_expression(1) | create_expression(2)
+        self.assertFalse(hasattr(expr, "__dict__"))
+
+
 class TestCompileFromExpr(unittest.TestCase):
     """ Tests for compile_expr. """