Issue 1868 dihedralrepr (#1869)

* fixes #1869 
* fixed order of indices in Angle/Dihedral/Improper repr
* tests for issue 1868

package/CHANGELOG

@@ -13,13 +13,14 @@ The rules for this file:
   * release numbers follow "Semantic Versioning" http://semver.org
 
 ------------------------------------------------------------------------------
-??/??/18
+??/??/18 richardjgowers
 
   * 0.18.1
 
 Enhancements
 
 Fixes
+  * Fixed order of indices in Angle/Dihedral/Improper repr
 
 Changes
 

package/MDAnalysis/core/topologyobjects.py

@@ -118,7 +118,8 @@ def __hash__(self):
         return hash((self._u, tuple(self.indices)))
 
     def __repr__(self):
-        indices = sorted(self.indices)
+        indices = (self.indices if self.indices[0] < self.indices[-1]
+                   else self.indices[::-1])
         return "<{cname} between: {conts}>".format(
             cname=self.__class__.__name__,
             conts=", ".join([

testsuite/MDAnalysisTests/core/test_topologyobjects.py

@@ -154,6 +154,11 @@ def test_angle(self, PSFDCD):
         assert_almost_equal(angle.angle(), 107.20893, self.precision)
         assert_almost_equal(angle.value(), 107.20893, self.precision)
 
+    def test_angle_repr(self, PSFDCD):
+        angle = PSFDCD.atoms[[30, 10, 20]].angle
+
+        assert_equal(repr(angle), '<Angle between: Atom 20, Atom 10, Atom 30>')
+
     def test_angle_180(self):
         # we edit the coordinates, so make our own universe
         u = mda.Universe(PSF, DCD)
@@ -174,13 +179,25 @@ def test_dihedral(self, PSFDCD):
         assert_almost_equal(dihedral.dihedral(), 18.317778, self.precision)
         assert_almost_equal(dihedral.value(), 18.317778, self.precision)
 
+    def test_dihedral_repr(self, PSFDCD):
+        dihedral = PSFDCD.atoms[[4, 7, 8, 1]].dihedral
+
+        assert_equal(repr(dihedral),
+                     '<Dihedral between: Atom 1, Atom 8, Atom 7, Atom 4>')
+
     # Improper_Dihedral class check
     def test_improper(self, PSFDCD):
         imp = PSFDCD.atoms[4].impropers[0]
 
         assert_almost_equal(imp.improper(), -3.8370631, self.precision)
         assert_almost_equal(imp.value(), -3.8370631, self.precision)
 
+    def test_improper_repr(self, PSFDCD):
+        imp = PSFDCD.atoms[[4, 7, 8, 1]].improper
+
+        assert_equal(
+            repr(imp),
+            '<ImproperDihedral between: Atom 1, Atom 8, Atom 7, Atom 4>')
 
 class TestTopologyGroup(object):
     """Tests TopologyDict and TopologyGroup classes with psf input"""