Added tests for kwarg storage behavior.

src/mdsynthesis/tests/test_treants.py

@@ -83,12 +83,25 @@ def test_set_universe_with_kwargs(self, treant):
             the Universe in the first place.
 
             """
+            # try out a basic, but nonexistent, kwarg
             u = mda.Universe(PDB, XTC, something_fake=True)
 
             treant.universe = u
-
             assert treant.udef.kwargs['something_fake'] is True
 
+            # try a kwarg with a value that won't be serializable
+            u2 = mda.Universe(PDB, XTC,
+                              something_else=mda.topology.GROParser.GROParser)
+
+            with pytest.raises(ValueError):
+                treant.universe = u2
+
+            # check that we get a warning if a Universe didn't store its kwargs
+            u3 = mda.Universe(PDB, XTC, something_fake=True)
+            del u3._kwargs
+            with pytest.warns(UserWarning):
+                treant.universe = u3
+
         def test_add_univese_typeerror(self, treant):
             """Test checking of what is passed to setter"""
             with pytest.raises(TypeError):

src/mdsynthesis/treants.py

@@ -108,8 +108,8 @@ def universe(self, universe):
             try:
                 self.udef.kwargs = universe._kwargs
             except AttributeError:
-                warnings.warn("Could not store universe keyword arguments; "
-                              "manually set these with ``Sim.udef.kwargs``")
+                warnings.warn("Universe did not keep keyword arguments; "
+                              "cannot store keyword arguments for Universe.")
 
             # finally, just use this instance
             self._universe = universe