Only create basal ganglia and thalamus if required.

Fixes #42.

nengo_spa/actions.py

@@ -1,4 +1,7 @@
 """Parsing of SPA actions."""
+
+from itertools import chain
+
 from nengo.exceptions import NetworkContextError, SpaParseError
 from nengo.network import Network
 from nengo_spa.modules.basalganglia import BasalGanglia
@@ -211,8 +214,13 @@ def _parse_and_add(self, parser, action, name=None):
         if name is not None:
             self.named_actions[name] = ast
 
+    @property
+    def bg_actions(self):
+        return [a for a in chain(self.actions, self.named_actions.values())
+                if isinstance(a, Action)]
+
     def build(self, bg=None, thalamus=None):
-        needs_bg = len(self.actions) > 0
+        needs_bg = len(self.bg_actions) > 0
 
         if len(Network.context) <= 0:
             raise NetworkContextError(

nengo_spa/tests/test_actions.py

@@ -198,3 +198,13 @@ def test_provides_access_to_constructed_objects_of_effect():
             else:
                 raise AssertionError("Unexpected object constructed for Bind.")
         assert n_connections == 2 and n_bind == 1
+
+
+def test_bg_and_thalamus_only_created_when_required():
+    with spa.Network() as model:
+        model.state1 = spa.State(16)
+        model.state2 = spa.State(16)
+        bg, thalamus, _ = spa.Actions('state1 = state2').build()
+
+    assert bg is None
+    assert thalamus is None