Added fix for Issue #532

Prevents addition of AtomGroups from different Universes

package/MDAnalysis/core/AtomGroup.py

@@ -533,6 +533,10 @@ def __add__(self, other):
         if not isinstance(other, (Atom, AtomGroup)):
             raise TypeError('Can only add Atoms or AtomGroups (not "{0}")'
                             ' to Atom'.format(other.__class__.__name__))
+        # Dodge property to avoid NoDataErrors
+        # Will then allow addition of NoneUniverse to NoneUniverse..
+        if not self._universe is other._universe:
+            raise ValueError("Can only add objects from the same Universe")
         if isinstance(other, Atom):
             return AtomGroup([self, other])
         else:
@@ -1074,6 +1078,8 @@ def __add__(self, other):
         if not isinstance(other, (Atom, AtomGroup)):
             raise TypeError('Can only concatenate Atom or AtomGroup (not "{0}") to'
                             ' AtomGroup'.format(other.__class__.__name__))
+        if (self and other) and (not self.universe is other.universe):
+            raise ValueError("Can only add objects from the same Universe")
         if isinstance(other, AtomGroup):
             return AtomGroup(self._atoms + other._atoms)
         else:

testsuite/MDAnalysisTests/test_atomgroup.py

@@ -2191,12 +2191,21 @@ def add(x, y):
 
     def test_add_mixed_universes(self):
         # Issue #532
+        # Checks that adding objects from different universes
+        # doesn't proceed quietly.
         u1 = MDAnalysis.Universe(two_water_gro)
         u2 = MDAnalysis.Universe(two_water_gro)
 
         A = [u1.atoms[:2], u1.atoms[3]]
         B = [u2.atoms[:3], u2.atoms[0]]
 
+        # Checks Atom to Atom, Atom to AG, AG to Atom and AG to AG 
         for x, y in itertools.product(A, B):
             yield self._check_badadd, x, y
 
+    def test_adding_empty_ags(self):
+        # Check that empty AtomGroups don't trip up on the Universe check
+        u = MDAnalysis.Universe(two_water_gro)
+
+        assert_(len(AtomGroup([]) + u.atoms[:3]) == 3)
+        assert_(len(u.atoms[:3] + AtomGroup([])) == 3)